Non-cuben WP/B fabric is significantly cheaper, though still more expensive than Sil. GORE-TEX brand 3-layer WP/B fabric can be had for about $25/yd from Rockywoods. Other brands and generics can be had for as low as $8.50/yd on sale but an average of about $16. While still expensive, it's certainly within reason for someone to spend ~$150 on a 4-season WP/B shelter (that was the original intent right? I can't find the original thread).
I guess the biggest balancing act would be weight. WP/B fabric is generally 4-5oz per square yard, where 1.1 silnylon is about 1.4oz. A fully WP/B shelter could easily be 4lbs making it too much gear for some (most?) people.
So, I'll modify my second question, since I think this is what I really wanted to ask anyway:
Assuming that weight and cost weren't factors, are there technical reasons why a fully WP/B Dirigible wouldn't work?
ETA: Forgot to say, Thanks for the response Catavarie!