|laziness, impatience, and hubris|
I know nothing about IIS, so I am going to contribute basically nothing on-topic for this node. Sorry.
However, I do use Mason extensively. Your third bullet above talks about finding a system that's easy for designers to work with.
I'm working on a paper called "Working with Designers and other non-Technical People". I haven't decided whether to submit it for the ApacheCon or YAPC. In my experience, Mason is far and away the best solution for parameterizing large websites where multiple people will be doing work on it.
Mason's lead programmers are very focused on making Mason non-mod_perl specific, and I do use Mason for email (one of these days I'll post my vi scripts that let me do all sorts of fancy Perl things in email and news) so it's certainly workable outside of Apache.
TT is also a very viable solution -- it will probably be faster than Mason, but I would still claim that Mason beats it in terms of the working with non-geeks factor.
About a year ago, Mason was in vogue in the Perl community; everyone talked about how much it rocked. These days, TT has mostly replaced it as the one to buzz about. I use both, and they both have their plusses and minuses.
Finally, you might consider Zope and friends. While tied closely to Python, Zope is not only Python -- most of its features can be used without knowing any snake-speak at all. And it has some great collaboration features that no Perl product I know of has matched yet. I also know that Gisle has been working on making Perl work in Zope, which would make it an even more inviting choice.