Of the two approaches,
I think the latter would take less time. Since pnsd's current issues have stumped me (debugging query parser, implementing list builder and ad_form in pure tcl), I can't take a guess at how much longer it would take. I think they're solvable, but for someone closer in tcl ability to Michael Cleverly.
For adding FastCGI support to AOLServer, there are a large number of fastcgi implementations, including several in C (see fastcgi.com) and one or two in TCL
Looking at the example code, I'm guess you're talking < 1K SLOC to implement this, most of which is already written. It is probably on par with implementing a new database module.
I don't think that's 3-6 months, but maybe if AOLServer is really hard to hack on. Which I doubt.
As Patrick points out, this approach puts AOLServer on par with how most Java Servlet engines run -- out of process and speaking through a socket to some in-process connector. However, from my own experience with fastcgi/apache, configuration will be much closer to mod_rewrite+mod_proxy than mucking with xml config files to make your java app server happy.
Crucially, going this route also gets you IIS support for free (where mod_proxy is not an option).
As to who can do it... I think anyone who could extend AOLServer would be qualified to do this.