IDEal can't launch UPX as an external app on my setup, from what I have tried, it just doesn't launch UPX and doesn't give any hint if it tried or not.
For those interested, IDEal can supply the UPX executable with a parameter of %LastExe%. If the filename or path of the last executable created has a space in it, you need to enclose %LastExe% in quotes: ie "%LastExe%"
...otherwise UPX will exit without error (v2.91, at least - I'm about to upgrade to 3.0)