REI Plugin Compatibilities adds support for JEI plugins to REI. In order to trick mods with JEI plugins into thinking JEI is installed, it presents itself as JEI version "9.9999".

Just Enough Calculation expects JEI >10 to be installed, which prevents the game from starting.

Since I don't know whether it's even possible to conditionally specify Forge mod dependencies, maybe the min. required JEI version could be set to v9.9999, and JEC's JEI compat could be loaded or ignored depending on whether REI-PC is installed?