This is true, as I can report: I proxy @ecoinstar to @ecoinstant and it works for witnesses and proposals.

In this instance, I used the proxy to automate unvoting those who approved the hf. But you are completely right, and I hadn't thought of it....now using that strategy would change all of my proposal votes....not ideal!

I use steempeak.com for proposal voting. steempeak really has their act together. Recommended to use with keychain browser extension.