On PS2-Versions before SCPH-50k, you already need to be in PS1-Mode before you swap.
So having an original PSone game in before starting uLE won't work?
Nope, since uLE is a PS2-Application and therefore you are in PS2-Mode, when you swap.
The method you described works, but only on SCPH-50k and upwards.
If you try it on older versions, it will hang on the PS1-Logo since older versions make a secondary check there, if the inserted disc is an original.
Hey, I just posted an alternative method to boot PSX backups with just an original PSX disc. Check it out here: http://psx-scene.com/forums/f77/%5Bguide%5D-ps1-backups-without-psxlauncher-gs-breaker-62232/