As has been mentioned, a mechanism for doing just that has been
developed and implemented. See http://www.cs.ubc.ca/doc/world/exec/intro
for details and patches to your browsers (the lynx patches are out
of date, but the Mosaic for X patches should work even on the newer
versions).
Briefly, it defines a new URL scheme "x-exec" which looks like:
x-exec://program_name/some_args
It looks to see that "program_name" is allowed by a WWW_EXECPATH
environment variable and if so runs it as a CGI script with
PATH_INFO set to "/some_args", etc. and displays the output
(which, of course, can be HTML, GIF, or any MIME-typed thing).
Now all you need to do is to patch your browser or convince the
browser writers to make it standard. Given that the URL working
group has decided that you don't even want to reference local
files, I suspect the best that x-exec: can hope for is de facto
standardization.
-- George