The only venues I'm familiar with are ENWorld.org's Play-by-Post subforums [enworld.org/forumdisplay.php?f=62] and RoleplayOnline [rpol.net/]. Both work just fine. Rpol.net provides a lot more options (groups, character-avatars, ability to send messages to specific members, a direct link to a character sheet...), but I find ENWorld to have a better, smoother interface and appearance.
I suspect recruiting for an Ars game would be easier at RPOL.net, although I'm not sure. ENWorld is large, and I myself had recruited two PbPs there. (You could always recruit here, on RPG.net, the Berklist and so on too.)
I'm afraid I won't e able to participate anyways. Too many other roleplaying (and other) obligations.
When I started one on RPOL.net, I got a ton of responses very quickly. I ended up with a larger group than I was expecting because of all the interest, but still had several players that had to be turned away.
Not abducted by aliens. I still lurk both here and on the Berkeley list. I got discouraged on the other list, both by the unwholesome tone of discourse and by my judgment that my deviant offerings were generally not useful to people.
I thought about writing up a magus or maga for the open call. But not Eilonwy. For all that I too remember her fondly, I don't know how to build her using canonical 5th edition rules, if only because I own none of the supplements. (IIRC, weren't the open call magi supposed to be created using the core rules?)
When I saw that you were thinking about running an AM5 game, I felt the stirrings of temptation, even though when it comes to PBEM games I ought to know better! If you change your mind yet again, let me know. I wouldn't run Eilonwy--she belonged to Tintagel. But there are other characters where she came from.