This may be a stupid suggestion, but perhaps it would be easier to write the srfis in a specialised markup format, and then generate both the docs and html/xhtml from that?
It's not a stupid suggestion at all. However, I (the editor) am very nervous about making any fundamental changes to the process that has served for twenty years, and HTML has been a requirement from the beginning. More practically, I'm sensitive to any proposal that adds work for authors or that might send us down a rat hole arguing about exactly the right formatting language. That could turn away authors who might otherwise be willing to contribute great SRFIs.
It's a really delicate technical-social balance.