Can't they use the RF API to emulate EU?
If you read the first two comment threads on https://www.reddit.com/r/feedthebeast/comments/3gg439/a_tale_of_joules_or_why_moving_buildcraft_to_rf/, this is possible from what I can see as the internals of the RF API are fragmented in such a way that it allows you to do some very interesting things (it breaks RF blocks into 4 types: producers, transporters, storage and consumers. Using these 4, you could create custom RF "systems" that are fully compatible with other RF producers, transport and storage, but can cause issues with consumers depending on how you go about things. From what's been said in that Reddit post, most RF consumers assume that RF will be a steady stream, however both BC and IC2 to my knowledge use a "pulse" system (EU moves energy around the network through packets of energy, and hence the mechanics derive from these packets, MJ just pulses energy through the network, however RF produces a constant flow of RF that fills up every storage block, including RF pipes, in the network).
TL;DR To my knowledge, you can, but doing so will cause your custom RF "system" to become it's own, that isn't guaranteed to work with other mod's blocks, essentially making the idea pointless anyways.
Mind you, I haven't used the RF API, only going off what's been said in that Reddit thread.