Forum OpenACS Q&A: OpenACS request processor, db_* API, and AOLserver
Several folks here and/or on the AOLserver list have talked about tweaking the Request Processor to integrate more nicely with other AOLserver filters, but AFAIK no one has contributed any such changes back to OpenACS.
It is feasible to make the OpenACS db_* API (and lots other useful stuff from the acs-tcl package) run standalone in AOLserver without OpenACS. I did that once long ago, for ACS 4.2. However, the OpenACS code currently does not come set up to work that way out of the box, you'd have to hack on it.
The OpenACS maintainers I've talked to about this are basically in favor of changing the Tcl code to work out of the box both with and without OpenACS (and ideally both in AOLserver and tclsh as well), it's just never been a priority for anyone to actually do. (Including me - I volunteered to do it sometime, but I've no idea when I might actually get to it.)