I think the problem is that the machine shop won't know what the reference is. In other words, the tables could be both high on the cutter side, but low on the other side and each side could still be flat. Does that make sense? If one or both tables were 'leaned' the wrong way, but flat, then you may not be able to adjust enough to make them co planar.
I'm not sure how you would grind them to be co-planar unless you kept the jointer together when grinding unless you want to go down a complicated path. I would grind one face flat and use it as a reference to grind the other face. You don't care about how level it is to the ground, but both faces should be level to each other. Also, if you are sending it in as one piece, you'll need to make sure that each side is very tight. After all, if one side moves during grinding, all your work is pointless. You'd need the tables to be parallel to the cutter length too unless you have some adjustment from the side nearest the user to the side opposite. If you could also send the jointer to the machine shop with the adjustments in the middle, that would be best.
Anyway, a gentleman above had good luck with an auto machine shop. I wonder if he took his apart or sent as one unit. Good luck and cheers!
Edit:
I would use the cutter body as a reference for parallelism along the Y dimension (along the length of the cutter body). Your beds need to be parallel to the cutter body. The blades can be adjusted, so I wouldn't use those.