Please don't create BIPs that don't have any actual
implementation behind them. Design discussion is fine but the
mailing list works for that.
If I were going to implement escrow transactions in BitCoinJ
it would not matter what was written here. I'd just implement
the design I thought made sense. If that design was later
adopted by others it can be documented and agreed upon in a BIP,
just like a regular RFC.
For what it's worth I would not attempt to send half-valid
escrow transactions through the p2p network, not even using the
overlay networks the protocol already supports. A correct escrow
protocol requires the seller to challenge the dispute mediator
with the public key to be sure they actually own it, and the
simplest way to do that is to leverage the existing DNS/EV-SSL
infrastructure with a "sign this nonce" HTTP request.
BIPs should not be a place for people to come up with
armchair designs, because a design with no corresponding
implementation is likely to be full of problems. Let's revisit
this once I can install some software on my laptop, my server,
and a friends server, and do a 3-way mediated transaction
between them.