Is there a standard name for a fallacy of the same form as an ad hominem, except that instead of denouncing the opposition, it praises the defense?
Typically an ad hominem ("against the man") fallacy of (ir)relevance is described as requiring abuse:
Bob favors Y. Bob is a lazy shiftless loser. Therefore, Y is false.
In a sense, before the fallacy, Bob is somehow neutral, (as are Y and not-Y); after the ad hominem, Bob goes down a peg in the listener's esteem, which for an uncritical listener seems to put Y down a peg too. Therefore Not-Y is "above" Y.
Relatively speaking it doesn't much matter if either Y goes down a peg, or Not-Y goes up one, so long as one of them moves in the desired direction. The fallacious goal being to put Not-Y above, which can also be done by praising the defender of Not-Y:
Bill favors Not-Y. Bill is a hard worker, and in 2012 was awarded the coveted Gilded Nostril. Therefore Not-Y is true.
Other than the relative motions involved, (analogous to addition with positive and negative integers), the invalid form of both is the same. In the field of public discourse, both forms appear to be equally common. Bureaucracies seem to use the praising form more.
Abstracting both forms into one, an irrelevant but prejudicial personal attribute replaces (or overshadows) a premise, which incorrectly prejudices listeners to accept an unsupported and therefore invalid conclusion:
Bilbo favors Y (or not-Y). Bilbo is naughty (or nice). Therefore, Y is false (or not-Y is true).