Right now, you're taking something 'x-exec:blah blah' and treating it
as a URL that executes something special.
Why not, instead of searching for "x-exec:", search for
"http://cs.ubc.ca/exec/". That is, build *into your client* the
special case that if you see "http://cs.ucb.ca/exec/blah blah",
instead of sending http protocol somewhere, you execute "blah blah" as
a shell command.
This doesn't clutter the URL space either. You might think it is
rather special purpose to put such an odd special case in your
software, but in fact it is fairly general.
#define DUMMY_HTTP_EXEC_SERVER "cs.ucb.ca"
if you like.
You'll get all the features you wanted with x-exec, and you'll also
get pages that function when you browse them with clients that don't
have any special purpose extensions.