Add an optional parameter prefix= which will prefix an also-specified group with a specified string. Also, maintain and produce separate lists of refs for each active prefix+group instead for each group.
example1: prefix=A group=lower-roman produces [Ai], [Aii], ...
example2: prefix=B group=lower-roman produces [Bi], [Bii], ...
I'm afraid this is literally what the group=… feature is meant to do. <ref group="A"> produces [A 1]. It's a bit unfortunate that the lower-roman property was implemented as a special case of the existing group=… attribute. But as far as I understand even that was intentional: a group can either have a prefix or use a different numbering scheme. Combining both is not supported.