Just as a data point, the Sage notebook (and similar projects like
codenode) essentially are Web 2.0 AJAX things relying very heavily on
JS. People love it. There are some people working with the UTMOST
NSF grant to try to create something that would allow interactive
behavior without even logging in as well.
So in some sense there already IS such an interface for Maxima (use
%maxima, or just pull down Maxima from the language choices). But
more to the point, this is definitely possible, though one would need
people who know a lot about those sort of web apps to do it.
I already like Robert M.'s
http://wood.mendelu.cz/math/maw-html/?lang=en a lot, turning this into
something a little more JS-menu-y sounds quite possible. You may also
want to look at how Maple does its 'Clickable Calculus' - I mean
concept, obviously not code, and that is Java, if I recall correctly.
Also, jsmath is being superseded by MathJax - same developers, but
better long-term infrastructure and support. MathSciNet already uses
MathJax. Sage doesn't yet, but will eventually; the license is
GPL-compatible.
Karl-Dieter