"we have established that you can, in some cases, use trace via serial,"

I think I've used this technique on every project I've worked on. One of the most significant benefits is that you are debugging on the target without any side effects from using emulators.

You can effectively set breakpoints in the code by waiting for RI, or you can go more sophisticated and take action based on the character received to test chunks of code individually. It's a very versatile technique, I'm surprised you don't use it. Naturally you have to consider any effect it might have on program execution or timing, but you have similar considerations with any debugging technique.