Well, I suppose that those discussions mainly were about what should be considered 'in check' or 'passing through check' in the presence of an Imitator. And that isn't really a subtlety of the Imitator, but of the checking rule employed by the CV. That is outside the scope of Betza notation, which only has the purpose of indicating pseudo-legal moves.

The main problem I see is whether the imitation is 'recursive', in the sense that it would be able to imitate a previous imitator. The same problem exists with move borrowing, btw. The way I currently implemented that in XBoard aims to be non-recursive. Because Odin't Rune Chess defined it that way, but also because recursive borrowing easily leads to infinite recursions, when two move borrowers can borrow from each other. You would have to keep track of which pieces you already borrowed from. Or better yet, which moves you already borrowed, so that you would never generate duplicat move. For XBetza duplicats are not a crime, though; you can for instance say KR or DR, where the first is sloppy, but a unique alternative for the second in unreasonably cumbersome.  For Imitation this problem doesn't exist, and I think the most common type would be recursive, imitating the last-moved non-imitator.

There also is the problem whether you imitate special properties of a piece (like promotion), or just the set of target squares. But this problem also exists with move borrowing or induction. E.g. in Knight Relay Chess you cannot promote a Pawn through an induced Knight move. (You cannot even move it to first rank, so this is really a confinement issue.) But promotion is again outside the scope of Betza notation, the latter just describes methods of locomotion. There are some CVs where the set of moves that allow you to promote is not equal to that where you can defer promotion. (In particular micro-Shogi, where you (mandatorily) only promote only on capture, but there also are variants where you can promote on a null move in the zone, while plain null moves are not allowed.) This would be most conveniently described by just giving a separate move description for promoting moves.

There is an article about 'Mimics' here on CVP, which I stumbled on when looking for Mimotaur. It also mentions a 'Strict Imitator', which imitates the actual move, rather than the piece type. I would say that is just another atom.

I try to remain pragmatic; Betza certainly was, with his z and q modifiers. If it is simple and useful, it is worth having, even if it is not perfect in the sense that it can do everything. The way XBoard uses XBetza move descriptions makes it already useful to describe approximate moves, so that it knows what is certainly not pseudo-legal. Having a recursive imitator in the toolkit would certainly be helpful.