Yes, I'm pretty confident the 20V solenoid housing will work without having to cut the slot for the oil return. Again, the oil return slot is for the oil in the hollow rocker shafts to return to the oil pan. It's like a very weak oil pressure relief valve.
When the solenoids are activated, the oil travel through the rocker shafts and pushes the rocker arm pistons to activate the big cams. So what happens to the oil pressure when solenoids are off? It will escape through the rocker arm oil squirter and through the return oil valve in the solenoids. If you use the 20V solenoid housing on the 20VE head, some of the oil can't return to the return oil hole in the head and only through the rocker arm hole where it will eventually make it way back to the oil pan. The only possible long term problem I see with this route is if you don't kick in the big cams enough time. Like once a month or longer, and letting the engine sit for a year without usage. Then the oil in the return chamber in the solenoid housing gets cook and become gummy, since it just sits there.
Here's a picture of the solenoids. You will notice there are three different sections on the solenoids and each section correspond with the three chambers in the solenoid housing. All of the chambers are seal off from each other by the tight tolerance fit between the solenoid body and housing. Chamber #1 act as oil reservoir and holds the oil. Chamber #2(to cams) directs the oil to the rocker shafts and cams once the solenoid is activated and the plunger moves up to let the oil in. When the plunger moves up it will allow the oil to come into chamber #2 but it will also seal off chamber #3(oil return chamber) from the oil. The oil in rocker shafts will only go to chamber #3 if the solenoid is off (plunger move back down) and if the oil pressure is great enough to overcome the ball bearing valve with a very small weak spring(doesn't take much pressure to overcome it) in chamber #3. So in the VVL scheme, IMO, the oil return hole is not that important.