I managed to get OnlyKey working with Keybase but only partially, in the sense that it seems you can’t use hardware keys directly for offline signing. I assume this issue #1946 was never picked up @t11 ?
Therefore the best solution I have managed is locally generating and loading a GPG key, loading the pub key to Keybase and then signing device keys with the OnlyKey. Therefore one can use the Keybase app and functionality (things like chats and teams) with the trade off of having to use device keys (albeit signed from OnlyKey), but can use WebCrypt for direct operations encrypt/decrypt/verify knowing this is more secure using offline keys (the master PGP key).
If this is useful to anyone, I will write up the steps.