I thought about this the other day. If you look at the PCM console, its divided into different parts, and the bottom part were the buttons of sport chrono, PASM, PSM, rear wing, etc. are located is an independant strip (at least it sure looks like it from the outside). That strip could probably be removed and just be substituted with a another one that has the extra button, like the one that comes with non-chrono cars with PSM. This is probably what they do in cars without sport chrono that get a PSE retrofit, they just change that strip with another one with the extra button for the PSE instead of changing the hole PCM console.
The problem is that non-chrono cars don't get the sport button so you loose a button for the sport chrono but if you don't have PASM (-20mm cars) you can use a strip made for the PASM cars and use the PASM hole for the sport chrono. I.e. -20mm cars would need 4 buttons for the extra PSE button there (rear wing, sport chrono, PSM and PSE) so they can use a strip for the PASM cars that have: rear wing, PASM, and sport chrono. ANd use the PASM button for the PSE.