I've replaced all ns_xml calls in the xml_ wrapper procs with calls to tDOM and subsequently replaced all other calls to ns_xml with calls to the wrapper procs. There is no technical need to keep ns_xml around. The above mentioned outdated XML libraries can go too, as far as I can see.
I don't know about including the tDOM module with the other AOLserver modules. That is a question to be forwarded to Zoran et. al.
Please note that tDOM works will all AOLserver versions from 3.3+ad13 upwards. (Based on personal experience.) There is no reason to upgrade to AOLserver 4.0 on behalf of tDOM.
Worst case upgrade scenario would be to upgrade OpenACS, download, compile and install tDOM (from the AOLserver repository or elsewhere), replace nsxml with nstdom and restart the server.
With AOLserver 4.0 we can expect to see more and more regular Tcl packages making their way into OpenACS. Like tDOM they will be only losely tied to AOLserver (or not all) and it is to be expected that most of them will not be available from the AOLServer directory.
Wouldn't it be better to setup a resouce center/directory on OpenACS with links and instructions to the software components that (some) OpenACS packages depend upon?