Hi,
The code still needs to run on OpenACS 5.1, so adp_include doesn't exist there. But looking through the source I found this very simple solution:
eval [template::adp_compile -string {[formtemplate id="project_filter"][/formtemplate]}]
set filter_html $__adp_output
(replaced angle brackets by brackets because of restrictions here in the forum...)
This version doesn't cache the compiled code, but that performance penalty should be OK because it's quite a short piece of code.
Cheers!
Frank