> You can try the patch with the following settings and execute
> `xref-find-references` for example (M-?).

I tried to use your patch to implement grouping for read-char-by-name.
It helped to greatly reduce the size of mule--ucs-names-group
from 30 lines to just 3 lines that is a big win.

OTOH, it highlighted the shortcomings of amalgamating both
grouping and transforming in the same function:
mule--ucs-names-group doesn't need to provide transformation,
because the same affixation-function is used for both
grouping and non-grouping completions.  So for such cases
handling an additional arg `transform` is an extra burden: