Love the idea, but $360 for a premium username. Nah. Not doing that. Not for something like this.
(And I'd guess it doesn't much affect the subscriber boost you get from the people who rush to grab their preferred username or landgrab a primo username on a new service.)
Though I wonder whether a recurring charge for a premium username means that they'll ever reuse a username, renting it to someone else if the initial person stops paying. That could be a security problem (impersonation, a bit like with domain name registrations). And if the initial person knows that's a security problem, keeping them paying for that reason might be a racket. (Maybe this is not a big deal for a calendar service, but it could be for others.)
remember you can always get a free account with firstname-lastname or similar
"Premium usernames cost $29/mo. You can start with a non-premium username and then upgrade later."
[edit: reading up a bit, I'm really rooting for you!]