I would certainly restrict the bribing option to certain elections. For the ones in which each noble has just one vote, it would be very strange that you could fake their votes, given the prestige they count with (as long as the vote is not secret, which though IG-mechanics it is, I don't see as very medieval). When you have elections in where prestige or knights counts, then it would be easier, for you would simply bribe the low nobility of which the prestigious nobles has the support from (I suppose that is how the system works).
In any case, we have two different possibilities it would be nice to clearly separate: bribing electors (namely the NPC lower nobility, for the players you already can), or rigging the elections (and bribing the elector officers, who are probably not public employees without major loyalties as in modern democracies, but probably servants of the royal or a very powerful house), which should be severely riskier and with harder consequences. The first option is buying the will of the voters, the second is outright fooling them.
