I wouldn't worry about the finish too much. After all, who is going to be peering under the table to see how neat the finish is?
There should be no problem with the interaction, If you oil the lacquer, it will just sit on the surface and eventually be wiped off. If you lacquer the oil, it won't stick very well (so I would lacquer first, and oil after).
What I would be more concerned about, is the strength of the joint. Gluing to end grain is weak, and half of the plywood surface will be end grain.
I suggest putting some screws through the back of the cantilever and into the wood top (being careful not to go too far of course!). The problem with this, is that it will tend to split the plywood; therefor use round-head or pan-head screws, and screw onto a washer (to spread the load further). If you are worried about the visibility of this, you could hide the screw heads in a recess you drill with something that gives a flat-bottomed hole. Pocket screws would be perfect for this.
Alternatively, you could dowel/biscuit-joint the top to the legs. (Belt and braces would be both)