Actually Additions has several Jams that provide potion effects when eaten, but when placed in a sandwich, the potion effects are not applied when the sandwich is eaten. Is compat possible?

Unfortunately, compatibility isn't really feasible from my end. This is because the Jams call Item#onItemUseFinish instead of ItemFood#onFoodEaten to apply the potion effects. The latter is what vanilla uses for food potion effects so that's what I use, and trying to call Item#onItemUseFinish instead from my end could cause unwanted side effects.