This would necessitate either:
1) Adding a "see also" class to the HTML.
2) Adding a "See Also" section to all SRFIs that refer to other SRFIs.
3) Adding a separate "fixup" file for each SRFI with manual additions
that aren't marked up in the HTML. (Or editing the auto-generated
metadata file directly but I would try to avoid this, as it will be
hard to tell apart the automatic and manual parts.
Should the categories be visible in the SRFI document itself? They could
even be links to category pages (auto-added by e.g. Ciprian's script).