You might be in for a milk run. Postgres 7.1, and its associated support packages, are already ported; tcl8.3 is ported; perl is ported; and I'm quite certain I saw a port of aolserver3.2 a few week back -- but I can't find it now. Regardless, the port may be fairly simple. AOLserver already runs on Alphas, so it's 64-bit clean, and the S/390 groups are fairly active so I'm guessing help could be forthcoming.
In re: to your questions, there are a number of power & cost reasons to use a mainframe rather than a PC farm, but if you're dependent on some i386 closed-source stuff, you don't have that option. And of course, the hardware is absurdly stable (even after 20 years, I should point out. Though by that time you'll wish it wasn't.).
I've been trying to get some time on that shared IBM machine for a couple months now. I have an unhealthy interest in what AOLserver, on a mainframe, could do on the SpecWeb99. If you get the chance, show us some numbers!
