This question mixes the action of the will with the original desire or choice of the will to attempt to carry out an action over time.
- Humans can behave irrationally
- Irrational behavior by definition does not have constraints
- Behavior follows from willed choices
Then it follows that human will can make irrational choices that have no constraints (physical or otherwise).
It seems you are trying to put the will in a box it doesn't fit in - limiting it to a choice at a particular point in time among physical actions (similar to how the compatibilists define it). The problem with this is that when the will makes a choice, it is assuming that it will endure to carry that choice out. It doesn't limit it's decision making to a particular point in time. Our mind can also think abstractly and thereby have access to non-physical possible solutions to our dilemmas.
The laws of the universe, to the extent that they truly do exist, certainly limit our actions, but they do not limit our will. Now, granted, we "learn over time" and limit our own will through the normal feedback process of seeing the results of our actions and creating mental models of how the world works. So, sometimes we "imprison ourselves", but there is no requirement that we do so.
Therefore, my answer is that there is no physical limitation at all on the choice/desire of the libertarian free will (though the apparent physical choices often heavily influence our choices...)
If you wish to talk limitation, perhaps we need to talk about the limits of what the human mind/will are able to conceive.
Edit: It is worth noting, based on the comments, that the will doesn't even have to choose to address a particular issue that is presented to it physically. For example, people that are catatonic don't respond to sense stimuli at all. In social situations, we often change the subject or outright ignore the question or statement that we don't want to address. Human beings have a huge array of issues/problems/items they are working on in life, and while most of us typically set a priority on immediate physical issues (I would guess because they are typically an impediment to the longer-range, larger-scope things we are working on), that certainly isn't true in all cases.
Several detailed examples:
Example 1 - Walk through the wall
I can choose to walk through the wall in front of me. The result of that choice may be that I end up with a bump on my head, but the point is that I can make that choice and follow through with attempting to walk through the wall. The mind, whether casual or not, acts in an abstract manner that allows one to choose to attempt to perform physically irrational behavior.
Example 2 - Falling from a 20 story building
Someone falling from a 20 story building, actually has lots of choices for how to spend their time.
- They can scream, or not.
- They can attempt to slow their fall or somehow direct their airflow to land on a ledge a few stories down and perhaps survive.
- They can flap their arms, like wings
- They can close their eyes and imagine they are back in Kansas and there is nowhere like home
- They can meditate
- They can attempt to will themselves out of their current situation by mental power alone.
Granted, most of these will have limited to no power to change the physical situation their body is in (based on general understanding of our universe - cannot truly leave out solipsism, idealism, etc.), but the point is that doesn't affect their will to try to take action, or not take action, in various rational or irrational ways.
One thing I do know - it is highly likely that the subject will soon find out if the mind/will survives the physical body or not. Well, if it does not, they really won't "find out", but you get the point...
Example 3 - lifting a 50 ton weight
A better illustration may be a situation from the comments - lifting a 50 ton weight. First off, by free will I can choose to ignore it - why do I have to lift it? Second, I can attempt to lift it, or decide that wouldn't be worth the effort. Granted, I could try for an unreasonable amount of time, but who says the will is rational? I could literally kill myself attempting to move it (beat myself to death against it, exhaustion, etc.). I could even sit down and try to use mental power to move it.
Are most of these irrational? Based on generally accepted understandings of the universe, yes, and yet, I'm sure we could come up with examples of real people who have done similar things. Some examples can be found in the "Darwin Awards" - google if you are not familiar with these.
But the point is not that I will physically succeed in my willed actions, but that I can choose to take a course of action independent of whether the outcome is or appears to be limited by physical constraints
Beyond the irrational, I could will myself to move it, and thereby work hard to get the money over my life to buy the land the weight was on, and afford a crane that was capable of moving it. Moving that weight might be my life work, and I may or may not succeed in the end - but the point is I can make a "free will" choice (if one believes the libertarian side) whether to decide to attempt to move it or not, regardless of the physical limitation you say (and we all may agree) exists.