I would be happy to fix the issue myself, but don't want to duplicate the efforts of others.
Consulting my notes, it seems to me that the Scintilla lexer for assembly language could fairly simply be generalised so as to allow syntax details to be specified as configuration options. The result should be general enough to cover all variants of Gnu assembler, for example. I am particularly interested in ARM support, where the main issue is that comments start with @.
You might say whether the Geany project would accept such a change in principle, or whether it would have to come via Scintilla? The latter requires more bureaucracy, particularly because (at I understand it) Geany is pegged to a previous release of Scintilla to the current one. Is that right?