After off-list discussion with Hugo, I'm convinced that
adding a requirement for signers to derive participant ids is
unnecessary complication that only buys slight convenience for
coordinator, and this is not a net positive tradeoff.
And since the coordinator can choose to generate unique TOKEN
I think that it is better to issue individual TOKEN for each
participant. Otherwise it will be possible for one participant to
attack another (intercept and replace their xpub sent to the
coordinator).
It will also be convenient to have a public 'participant id', derived
from the token. It can be
