In Keith Flowers Gambit-C for Android, there is a "program.scm" file that contains:
(define (set-page content handler) (set! event-handler handler) (set-webView-content content) (show-webView))
and that's pretty much what you would do. The same file shows building and html string for the content, and adding a function for the handler.
http://apps.keithflower.org/?page_id=152
to download the source archive.
I believe Android supports html5, so I would play with the <canvas> tag and a bit of javascript (you can just dump the graphic you want into the webView content to try). Interaction will be limited so a drawing program may not be.. um.. a good thing to do, but basic stuff like editing the scripts, and pushing buttons is done is program.scm, and can be "borrowed".
Anyway, you don't "post", you generate "events" using javascript embedded in the content.
If your application started an "integrated" web server, you could "post" to that web server. But, I think the content/handler approach brings everything together nicely, and is preferable.
FredW