I love the rubber diaphragm idea ( there is a hole thread about ways to do this ) ! i think it would definitely work great with a 14 dollar servo! but im not so sure about a solenoid.
I think you would have to displace a fair bit of water each time but with a servo that would be easy, and the peachy software already has an event driven servo feature( usb serial --> arduion --> servo--> valve) so this is practically done!

as a bonus the same setup could be used to over come the break over problem in that the normal peachy will have with thicker resins.
weather printing with 3 liquids or not you will be building a valuable feature for your printer.

I can see a hydroscopic coating working wonders here also, good thinking!!
i think resin will act the same as water because the basic theory is that there is a layer of air being conserved in the coating
... https://www.youtube.com/watch?v=IfUaKXasdD4

I think your on to something Jstrack2, once you have posted pics of your diaphragm level changing reservoir working and id be happy to come back with some peachy software modified to signal an arduino every layer.