Update
The URLTrigger has been updated and approved for On Demand as of FogBugz 8.
Hi Sean,
URLTrigger version 1.0.4 is not compatible with FogBugz On Demand, however that compatibility is the next item on the todo list for URLTrigger.
It's difficult for me to tell you when, but URLTrigger is my baby, and I want it to grow, so hopefully it won't be too long. The reason that it's not yet available for On Demand is that there are all sorts of sneaky things which URLTrigger could be used for in an On Demand environment, and as such URLTrigger isn't quite ready for primetime. That said, we know the solution and implementation is pending.
URLTrigger is the only way to have FogBugz trigger the call, however if you were to implement a polling mechanism in an external app, you could use the FogBugz XML Api to query FogBugz for most of the same information that URLTrigger can provide you with, and then have your external app send the HTTP request.