OpenNSD provides much more than a running Tcl interpreter. It provides a rich API that's heavily used by the toolkit. In order to use something like TclHttpd you'd have to replicate the nsd API, i.e. every procedure called "ns_something" used by OpenACS.
That's in essence what mod_aolserver for Apache does - it runs a Tcl interpreter and implements the API, which lets the ACS run under Apache.
