I would think the documentation would be far easier to maintain in html
than in Lyx format.
    
I suspect Johan, Bernhard and others, who currently maintain it using lyx, would probably not agree.  But easier or not, it _is_ currently maintained (using lyx).

How about as plain Latex ? 

Then it could be converted to a single or multiple html file via latex2pdf ?
I believe lyx is a gui front end that produces latex, and the html copies are produced from that using latex2html and the pdf copies are produced using latex2pdf (with some caveat about certain fonts not being installed at sourceforge).  For example, if you look at:

http://sdcc.sourceforge.net/doc/sdccman.html/

and view the html source, you'll see the comments that it was generated using latex2html.



Paul