Comment 6 for bug 1736911

> What I like about adding a userTagProvider() API is that it wouldn't affect modules like asyn

To my mind, this sounds like something which can be added to eg. QSRV directly without
bothering with any change to Base.

I would caution that doing this lookaside blindly for all records could easily introduce another point of coupling (via a mutex) between high and low priority scan threads. This is something SLAC has been concerned about in the past.