I know that there can be issues when moving a working SD card between different RPi devices (e.g. Ethernet MAC address stuff) but what about different screens?
I presume that this is seen as a likely scenario for school use - where the SD card and maybe even the RPi is moved between home and school.
If someone has a requires different config.txt settings for the different screens then it will become a chore to have to edit it each time.
A thought - how about groups in config.txt - with the default as now (no groups) and then have groups based on detected RPi serial number (for people moving SD card between static RPis) and on screen id/connection type (HDMI or composite) for people moving RPi between static screens (home and school).