Saga continuing...*grin*. Looks like ad_verify_install isn't being run because packages/acs-tcl/tcl/acs-kernel-procs.tcl is failing a test in apm_bootstrap_load_libraries; the *-procs.tcl files are returning a $file_type of content_page from apm_guess_file_type, which means they don't get sourced. I'm looking at the regexps in there now...oh. Found it. Tricky. It's the 'www' in /var/www, (my rootdirectory is /var/www/openacs-4) which is the standard "root location" for web stuff on Debian, rather than our beloved /web. That's matching the 'content_page' regexp in apm_guess_file_type.
For now, I'll rename /var/www to /var/web, but it's something to keep in mind (for future Debian packaging, etc.).