IIUC the documentation to glyphless-char-display-control, any of the
character groups can be assigned any of the display methods.

First bug + patch:
Using update-glyphless-char-display to choose to display
variation-selectors as acronyms does not work since these codepoints are
missing from char-acronym-table.
The attached patch attempts to fix this and succeeds for U+FE00..U+FE0E but
fails for U+FE0F.

Second bug:
It seems that U+FE0F will not at all respect glyphless-char-display,
instead always showing as an empty box.
This I have not solved.

Version: GNU Emacs 28.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.24, cairo version 1.16.0) of 2022-07-21
Built from source, commit 5a223c7f2ef4c31abbd46367b6ea83cd19d30aa7

Regards,
Axel Svensson