The short answer to that is that I'm assuming for the moment that a Chroma 81 is placed between the ZX81 and the ZXpand and that switch 3 is set. That should work for UDGs and also give the maximum free RAM (I'm on the waiting list for a Chroma 81 but the documentation for it is clear and EO can be configured to provide a similar setup).
I've been assuming that UDG4ZXPAND boards would require an M=L version (to get RAM at $2000). However I cannot be sure. AFAIK, no new UDG4ZXPANDs are likely to be made (I've put myself on the SMR waiting list), no one wants to sell one and a PM to Andy Rea went unanswered and was subsequently either manually or automatically deleted. As I couldn't find any detailed documentation for UDG4ZXpand and, unlike Chroma 81, I couldn't directly contact the maker I left that configuration in limbo. If someone can point me to some definitive documentation that describes how UDG4ZXPAND (safely) interacts with ZXpand and/or Chroma 81 in all possible configurations then suitable M=L version/s could be produced for it.
That's not to say that at some stage a M=L version won't appear for the Chroma 81 - I'm still thinking about configurations to support colour (or, more accurately, have been taking a short break and will return to that soon). Anyway, at least the documentation currently does state that M=H is required and that, for UDG, there must be RAM at $2000 (which is possibly enough to suggest that Chroma 81 will work and UDG4ZXPAND won't).
Alan