Thomas,

I wonder if you've seen my proposal on how to make BIP9 more configurable:
https://github.com/sanch0panza/bips/blob/bip-genvbvoting/bip-genvbvoting.mediawiki

This could be extended with a coinbase signaling feature as you suggest.
This could include parameter information for forks which a miner is signaling, for coordination.

Currently I've not included something like this, but it might make a nice addition.
One problem is the limited space in coinbase for embedding data on the large number of possible independent deployments.

Regards,
Sancho