While you can't install custom plugins in FogBugz On Demand (e.g., a plugin you decide to write yourself this afternoon), plenty of thrid-party third-party plugins are available in FogBugz On Demand that are just turned off by default.
So, to answer your specific questions:
No. The plugins available in FogBugz On Demand are shared across all On Demand accounts. This means that while individual accounts can enable or disable plugins as they choose, a plugin cannot be made available for just a single account and not others. If you would like to see your custom plugin on FogBugz On Demand, you would need to submit it for approval (i.e., make it useful for On Demand customers in general, not just you).
Installing/enabling approved plugins in FogBugz On Demand is simple: Admin -> Plugins -> View Available Plugins -> Click the link under the "Installed" column next to the plugin you're interested in if it's not already installed for your account.
As far as updates, I can tell you for a fact that FNN has been working with our QA team to get an updated version approved for FogBugz On Demand and should be deployed in the coming weeks.