So I ended up asking this in my Pre 3 craigslist poaching thread, but want to ask direct...
Does anyone know a way to force the screen that lets you enter the SIM unlock code to come up
without using a SIM from another carrier?
In my case I picked up an AT&T-locked Pre 3. Used Dogbert's method to get the code. He notes that
if the SIM unlock code provided doesn't work, then you have to repeat some steps with the extra --writeBack option.
The problem is, I have a Straight Talk AT&T SIM so the Pre 3 already worked on my SIM. So I can't get to the screen to try entering the code.
If I end up traveling to Europe I'd like to know it's unlocked, but have no way to confirm the code will be accepted in advance. And no, I don't know anyone on T-Mobile to try their SIM either.
Thoughts?

Checking the forums only one person seems to have asked while others just resorted to digging up a different SIM. Is it really impossible to reach that screen without an off-carrier SIM?