Take a look at the existing fractal explorer in Filters-Render-Pattern.
I think this basically does what you want. It doesn't support arbitrary
functions (I bet you could code in your own if you really wanted to), but
has quite a few options, and I imagine it could produce (with help from
the rest of the gimp) any of the images UltraFractal can make. The author
has a quick tutorial here:
http://www.multimania.com/cotting/fractalexplorer_en.html
(it's for an older version, but it still helps).
-James Smaby