That's a really good point. I think what we'll do is, prior to each election (except the first one), we'll call a quorum. If you aren't available, we'd just call you temporarily inactive (and, obviously, ineligible for any positions). I will, of course, listen to other ideas if somebody has one that's better.
I think the best (but labour-intensive) way of doing this is just to divide the party's votes in the same way as its actually voting members.
Example (Conclave only):
The Alliance: 10 seats
UAI: 30 seats
New Society: 20 seats
Imperialists: 40 seats
All parties vote on a certain education reform;
The Alliance: 3 votes in favour (3 votes)
UAI: 1 vote in favour, 1 vote against, 1 abstention (3 votes)
New Society: 5 votes in favour, 4 votes against, 1 abstention (10 votes)
Imperialists: 1 vote in favour, 3 votes against (4 votes)
Each party's seats would then be distributed according to the player votes;
The Alliance: 10 seats in favour (10 seats)
UAI: 10 seats in favour, 10 seats against, 10 seats abstain (30 seats)
New Society: 10 seats in favour, 8 seats against, 2 seats abstain (20 seats)
Imperialists: 10 seats in favour, 30 seats against (40 seats)
Result: 40 seats in favour, 48 seats against, 12 seats abstain
That example makes me wonder; what if a proposal has a relative, non-absolute majority?