[For reference, the proposal is to use db for the new version of utmp
and lastlog.]

> Our current db (v1) has locking problems, and the v2 licence isn't
> "friendly" at the moment...

To the best of my knowledge, db version 1 isn't really intended to
support concurrent access to a database.  But that doesn't have to be
fatal for multiple writers; just lock the file before updating it.

How to synchronize writers with readers is a harder problem, since the
readers are not all root and you don't want non-root processes to be
able to interfere with login(1).  In most cases the utmp record is
less than one block and there's no race, though.