The only thing that comes close to web-based Ultima anything is
http://www.lordofultima.com/
While the concept of a web browser -based UO client sounds neat, I bet it would be a real bastard to code in AJAX
You are right about the other shards. I've heard and read a few places that the average # of all free shards... at least those which are using RunUO (Pretty sure there more RunUO shards than Sphere shards) is only 8 people.
With Obsidian though... I think once stuff is straightened out, the shard can draw many players young and old on the premise of "Oldest running shard" and "oldschool shard". It's just the "oldschool" part that has kind of been up for debate with some of the changes to Obsidian shard.