Raised this issue on Ender IO git first, got this response:

I have just gone through the BC code and determined the cause of the issue.
It appears that BC Next is in the process of changing the MJ power API yet again. It appears that so far, only the quarry has been ported to the new system.
Once this new API has stabilised and has been applied across the BC code base I will add support for it.
You could try raising an issue with the BC devs and see if this is intentional. The cause of the issue is that the Quarry no longer implements 'IPowerReceptor'.

BTW, I just extracted the interface to interact with the new thing, on https://github.com/BuildCraft/BuildCraft/blob/NextGen/api/buildcraft/api/mj/MjAPI.java. This should hopefully make the migration easier. Note that the previous API will stay in 6.0 and probably 6.1, and that the BuildCraft engines and pipes will carry power to both.

Ok, relayed this info over to the ender io git

There's actually much more than the quarry that is migrated to the new system. Builder, Filler and pipes have been too. I think migration will be completed by 6.0.8. Sorry for the disturbance in your work.