I can think of 2 basic solutions :
- 2 versions of the initial save (will make the SGotMs very complicated, but seems doable for GotM and WotM)
- using the 3.13 CvGameCoreDLL.dll files for everyone. Even those without BtS would have to DL the file and overwrite the 1.74 file. This could be made by a 1.74b patch for vanilla.