In planning for Wikimedia PA, we tried an approach that was a little more variable, to account for the fact that we would start off very small, and could eventually become very large. Defining a quorum as something like "10% of active dues-paying membership or 10 persons, whichever is less" accounts for the fact that the board will grow as the community grows.
I think you mean "whichever is greater". The model docs already have the 10% bit in them, you just have to fill in the blank for the "or [ ] persons" bit, that's where I'm suggesting 10.
TD suggested we insert a provision explicitly allowing for IRC board meetings. Again this seems sensible to me, but we have to be careful legally.
Again, in WMPA we had allowances for this too because it would be so difficult to host a meeting at a place where every PA resident could easily get to. We allowed for members to participate in meetings via internet, telecommunications, or via authorized proxy. We also allowed for these things to count towards a quorum: So people who joined in on IRC or on a conference call or something, or who authorized a voting proxy counted towards the quorum. This also helps to eliminate the possibility that an inconvenient meeting place/time will disenfranchise your members.
You probably won't want to be as liberal as we were trying to be in this area, but it's food for thought.
I think allowing people to contribute to general meetings remotely would be a bad idea, it's just too complicated to organise (you just know you'll spend the first 20 minutes of the AGM saying "can you hear me now?" over and over). Board meetings should allow any reasonable form of participation. Allowing proxies is a legal requirement since the Companies Act 2006 and it already in the model articles.