Interesting. One complication with that scheme is that ther is no convenient place to put caching. If it goes between Pen and Zope, you need many instances of your cache. If it goes between Pen and the clients, Pen's client-clustering is defeated because it sees only one client.
Postgresql takes a similar (multi process) approach to achieving stability as apache. see pg 4 of http://developer.postgresql.org/pdf/transactions.pdf Think shared memory caching behind the Zope worker processes ;-) And web server / proxy shared memory caching in front of the Zope worker processes. Of course, for stability, there is also the possibility of doing a OpenBSD style audit of the entire Zope (and possibly python) code base to make the code as close to perfect (and therefore stable) as the OpenBSD OS is ;-) Volunteers? Adam