((I wouldn't object to BBB voting.
In other tangentially related ideas, perhaps BBB could introduce laws and reforms to be voted on, beyond what the presidential candidates say. These would be coming from various elements of Congress - either BBB seeing that there's some support for a novel idea among non-president characters in the AAR (perhaps we could suggest legislation here: if it got only minor support, it would just be killed off in committee or something. If it got more major support, it would be said to go on to a general vote, represented by BBB placing it up to vote for in the next voting session) or from all the congressional members we aren't playing. For example, in this instance there would definitely be Senators (and probably are a few characters already here) who would introduce legislation contrary to the president's aims - and something like a Missouri Compromise or a One-Free-State-One-Slave-State deal would gain quite some support. In occasional cases where the presidential agenda was this (abolition, certain labor issues, etc.), this could be a way of moderating the specific issues which would be less probable, without being completely "dictatorial" (not that we, or at least I, mind that). If it was really thought that we all might sway more towards a (at that time) radical and improbable idea, some votes might be added toward legislation against it (or against legislation for it) to represent congressmen who did not have our hindsight. We could still vote it through, but we'd need more than just a slim majority.
Speaking of majority, it might be useful to count some of these major changes as something closer to Constitutional Amendments or other things requiring more than a simple majority. I don't think we could get a 2/3rds vote to abolish slavery here.
Another thing is that we don't have as much incentive here to avoid controversy as congress did at that time. Perhaps some hints and explanation of possible repercussions would help sway us. Maybe not a full out secession at the moment, but maybe a coup or a revolt* (at least some militancy). More probability of counter legislation crippling aims (for example, someone saying "complete abolition" would get legislation thrown against them that would stop them and hinder anti-slavery movements, while slowly using anti-slavery and mild pro-abolition actions would perhaps be more successful in eventually removing slavery), some chance of overthrow... that might moderate policies a bit.
*I can't imagine a revolt actually threatening the government in Vic 2, though, unless it was very big (and then it's just frustrating the other aims - such as military expansion, budget balancing, etc. [and maybe this could be emphasized. The president's agenda would slow down in game whenever there was a major revolt, thus encouraging presidents to avoid revolts] - it's not too hard to keep the government from changing unless militancy and other revolt factors were just thrown through the roof). It would make a good propaganda thing for later campaigns, though. A revolution actually succeeding would be cool, though. They could place in power the person closest to their aims (perhaps a popular statesman, the losing candidate in an election, or just someone with similar ideals). ))