Not directly what you are asking for, but have you seen this macro?
At least it allows you to quickly swap Safari/STP as the http default browser, without the aggravating confirmation dialog.
In the thread scroll down a bit for a more recent version.
You could trigger the macro from some other macro that you are using for your specific tasks. That way it would be full-automatic.
Or, if you can nail your specific tasks to certain URL schemes (other than http) then you could also simply declare STP the default browser for those schemes.
Ahem, here is the most recent version (for KM8), which I obviously forgot to post in the other thread: