Hi, Ralf --
I think that the advice you have gotten from Fare and Robert Brown agrees with my experience (with another application that involves a translation from some alternate input form to common lisp source).
I have been working on an article for ILC that would describe how to extend the ASDF protocol. This is a process that was never very well documented, and one which has changed with the extensive changes (rationalizations) to ASDF.
Alas, I wasn't able to make ILC this year, which deflated my motivation for a few months. I hope to get back to it this winter.
Please post to the list about any challenges you encounter, or any ways the advice you have received so far was inadequate.
With luck, we'll get something good to make this easier for everyone!
Best, R