I made some patches for 3.5.1 starting from work done by Pierre Asselin for 3.4.2. You can find the results at
http://empoweringminds.mle.ie/openacs/ad13/ I'm not actually using those sources and would recommend going with 4.0 instead.
I posted a list of remaining AD13 vs 4.0 issues to the AOLserver mailing list the other day and got a few responses from OpenACS'ers. In addition to the above:
1. The ns_uuencode patch #474259. I should submit an updated diff. OpenACS falls back to a Tcl proc if this patch isn't there. The AD13+OACS implementation is broken.
2. AD13 has byte range request support (a HTTP/1.1 feature). There was a concern that, if added, this feature should be made configurable to prevent abuse by download accelerators.
3. Small additions/bugfixes in AD13. I'm not sure which of these are needed: ns_db stats, WarnEmpty parameter for database pools, SIGSEGV fix for Linux, 401 redirect in nsd.tcl fix, etc.
4. Don reports that you can't virtual host two OpenACS 4.x sites currently because nscache maintains one giant global cache for everyone, so the two instances clash over their "util_memoize" cache.
In a sense we haven't lost anything because you couldn't virtual host at all in AOLserver 3 without resort to solutions such as that made available by Jerry Asher, solutions that I assume still work?
aolserver/tcl/tclcache.tclin 3.3+ad13. That's not in 4.0 yet either.
However, that's a performance issue only, everything will work without it, and it would probably only be important for high volume sites. (So far, no one as really characterized the performance difference.)