If memory serves correctly from when I had to pass Microlight Air Law as it was then around 1992, there used - pre JAR-FCL - to be cross-currency rules. I think that of the 5hrs in 13 months needed on Gp.A, you could do 3 hours in 3-axis microlights.
Extending from that - doing the total hours for currency (whichever route - the 5/13 or 12/24) to include a minimum of say 2/12 in any particular class you want to retain minimum currency on would make sense.
I suppose the other way one might do it would be to require a total of UK 5/13 or JAR 12/24 in any class to keep your license legal, then expand slightly the 90 day rule; i.e. to fly with passengers in any particular class you'd have have to have flown 3 take-offs and landings in the last 90 days in that class - and that could include microlights, SEP or presumably SLMG if you've that rating also.
A third option might be 12/24 total and an hour with an instructor in each class that you want to remain current in - albeit that gives you a problem with the BMAA's desire to not adopt that particular piece of JAR.
P