I've launched the 7AT at the dragstrip by holding revs with the footbrake before, it's not an issue, before or after UpRev. There's a stall limit though, and beyond that you don't want to overheat your fluid, so it's a pretty low rev value for a pretty short time before launch. I want to say I was using somewhere around 2.1K rpm for just long enough for the final bits of the tree. If the ECU's even involved (as opposed to natural stall on the convertor), it's just protecting the fluid temp in that case.
All of that's not really relevant to my issue with UpRev being slow for me at switching from a low-rev/mph-limit map to a regular map, though, which happens all the time with my security map. I don't have any other issues with my tune. I just assumed this is how everyone's map switch worked and that there was some inherent reason for the delays, until I saw this thread about map-switch-launching. It's not a tune or engine or transmission issue, it's a rev limiter issue. It limits exactly like the rev limiter always behaves (but often at a different RPM, e.g. something between my low security limit and the new full redline of the normal map).
Then again maybe I'm unusual in being able to map switch and get out of the parking lot and trying to hit the upper rev range in a very short period of time, and other people are just slow enough out of the gate to not notice the delay
ETA: I also haven't downloaded my UpRev updates and reflashed the car in several months (well I just did a few days back, but haven't used the security map since), so it could be that the delay is something they've solved more recently.