> As for formal lexical specification, I don't think you need to worry
> about implementation-specific
> extensions as long as the spec won't conflict with such extensions.

It's not really possible to take such those conflicts into account
without adding dedicated remarks about each implementation. And an
implementation might change its syntax in future versions in a way that
obsoletes the remarks.

I checked the R6RS and R7RS syntax sections and didn't find any formal
provision to extend the number syntax (in the spirit of "please add your
extensions at these points in the grammar"). Maybe it's not even
possible to provide such syntax hooks, since any part of the number
syntax could be useful to extend for some purpose or another.