Using a FormBuilder in an ajax response?

I'm building my forms with a custom FormBuilder subclass. Now I want to
update parts of the form through ajax requests and therefore again need
a FormBuilder to build the response.

The form in question contains a partial that takes some parameters. It
is the content generated by this particular partial that I'd like to
replace using RJS with content generated by the very same partial, only
this time with different parameters (i.e., local variables).
Preferably, I'd do all this in an inline update, simply calling
render :update. However, I don't see how to instantiate the
FormBuilder. The only workable solution seems to be to wrap another
partial around the partial I'm really interested in and put a suitable
form_for in the outer partial. If possible, though, I'd like to avoid
such a wrapper partial.