I believe there are significant differences between the Hindu notion of Brhman and Dharmakaya based on reading the responses of Buddhists from tibet and east asia, as well as direct questioning of tibetan monk and a Sri Lankan buddhist who studied Mahayana buddhsim. I also think many of these differences can be seen in the fourth chapter of the complete translation of the tibetan work known to the west as the tibetan book of the dead. This chapter discusses the ultimate nature of mind and is an intoduction to the teachings on how the intermediate states between death and the taking on of a new birth can potentially be used the acheive enlightenment. It is a good read if you have a good translation. In it you will see the ultimate nature of mind being desribed in ways that Brahman is never described. For example, the Hindu Vedanta School describes Brahman "one without a second." In the fourth chapter of the translation of the book of the dead describes the ultimate nature of mind as free from the extremes of unity and multiplicity.
Other important differnces include the fact that some tibetans atleast do not accept the dharmakaya as a subtratum which is the ontological reality upon which the illusion of the world rests. That is how Vedanta understands the relationship between Brahman and the world we experience. In contrast, the buddhist view is that the world illusion is a creation of the unelightened mind which imputes a solid reality to things which are infact empty. This is true for mind-only viewpoints as well as madhymaka traditions. In Hinduism there is an ontological reality outside of our mind which we, in out ignorance, misread as a collection of discrete, finite, objects. In buddhsim, the idea is there is no truely existent extra mental ontological reality upon which we project the appearance of a universe of finite objects. Instead, we either impute an ultimate reality onto a complex network of causes and conditions which are constantly in flux, or in the mind-only traditions we are exerienceing creations of our own mind which are "internally" generated. Again, in contrast the Hindu notion is one of misreading the true nature of extramental, infinite and truely existent reality.
Another difference between the two notions is that Hindu view is almost fanatical about there being only one unigue Brahman/Atman. Thus, the constant emphasis in Vendanta that Brahman/Atman is one without a second. This is not the case in Tibetan circles. I asked a mind-only tibetan monk who spoke at Harvard University Divinity School about this. I asked if the historic Buddha's Dharmakaya was the same as mine. He said it was not. He then cautioned me that it would also be incorect to say that there was a dharmakaya that belonged to Shakyamuni buddha. I interpreted this to be a caution against the worldly temptation to grasp onto a ultimately existent self known as the "historic buddha." Other tibetans have contrasted dharmakaya and Brahman/atman by saying dharmakaya is not a subtratum underlying the world illusion and dharmakaya is not a "unique" reality as is Brahamn. It is true, there is a common notion out there that all buddhas share the dharmakaya. The dalai lama denied this in a book I read. I think the book was called "The Buddhanature." So at the very least dharmakaya differs from Brahman in the sense that at least many buddhist do not accept it as a single extramental substance. Apparently many well informed buddhists reject this idea of a single dharmakaya shared by all buddhas
. On the other hand, it is important not to be too gungho in our attempts to grasp a view about the ultimate nature of mind since if we truely understood this, we would already be a buddha which I certainly am not.
Other differences were pointed out by D.T. Suzuki. He pointed out that Vedanta understands Brahman in a way that sees its perfection precludes any activity. Brahman/ATman is an ocean of bliss. It is perfect in itself. Thus, the final truth about Brahamn is that it can't be viewed as acting at all. It is perfect in itself. What else does it need to do. Similiarly, Brahman is also perfect knowledge itself. It can't cognize anything else other than itself. If it did, it would be experiencing imperfect knowledge, that is some type of illusion. Any thing we experience that looks like Brahman acting or being aware of something outside of its own perfection is in fact an illusion. We as unenlightened jivas "see" saguna Brahman creating the world, or giving the "grace" of jnana to jivamuktas who escape the world illusion. However, in truth Brahman did none of those things. D.T. Suzuki contrasts this with Dharmakaya. His understsnding of dharmakaya, like that of many far eastern buddhists, does seem to accept it as a single reality. In that sense it may be similiar to Brahman. However, he points out that Dharmakaya is the Buddha who is quite aware of the suffereings of sentient beings and all other aspects of the universe in which they live. As the dharmakaya is in fact the body of the buddha who teaches sentient beings how to end their suffereing, the dharmakaya is also constantly taking action on our behalf. These are essential qualities of Suzuki's notion of the dharmakaya (as far as I can see). It is meaningless to imagine a dharmakaya that is not the union of wisdom and compassion in its "essence." (I know techically "essence" is never the best term to use when talking buddhism. Forgive me.) After all, the "essential" characteristic of the Buddha is that the Buddha turns the wheel of the dharma out of compassion for our suffering. Thus, in its "essence" dharmakaya is aware of quite a few discrete realities and is constantly acting on the behalf of those realities which are sentient and suffering.