I would consider making serious threats. Sometimes you need to scare people into posting.
Do a few things:
1. Send around a CM threatening to kick anyone who does not post in a roll call thread in the forums and/or reply to your mail. (Your response will be to kick between 5 and 10 players - preferably smaller players, they give impact while not costing the tribe much).
2. Scare people into participating. Threaten people with ennoblement like inactives if they don't participate.
As a last measure, threaten to leave your tribe, join another and disband the tribe in its entirety and make some top priority noble targets in your new tribe (bring the active members with you). I know you said not to mention this, but sometimes its beyond the point of no return.
I once led a tribe, and because I was carrying so much of the 'weight' and the driving force behind warring efforts, I decided to arrange a merger with a long time ally. We only had ~40 members, while they had the main and academy tribe of about 150 members total. Quite a few members were inactive as well.
Sometimes you have to let go and move on. You can try and kick, though you may end up with a tribe of 1.