Gauche has a reader hook for #!identifier, though it's not documented yet.  Using it, you can

write a source code in different syntax; I've written an M-expression reader so that

you can write code in M-expr, e.g.:

One of my little hobbyhorses is decimal floating point . In fact, I keep thinking, if I had time, that I'd like
to write a SRFI on this subject. This can all be done without changing the language syntax, except it would
be nice to have decimal literals, e.g. #m13.75. I don't know if this will ever catch on (decimal arithmetic is
a lot easier to teach than binary), but it would be nice. Presumably the same mechanism that would allow
for underscores in numbers would allow this.

Of course, there is the usual name-clash problem here, as there is with SRFI 10's proposed define-reader-ctor.
We might have to write

   #!com.femtodyne.my-identifier

to distinguish it from

   #!com.evilcorp.my-identifier

I do think that a SRFI on this subject would have to address this point, and either provide a mechanism or
guidance.

-- v