IMO, the best system would be to let anyone who's a beneficial owner cast a vote with the rest of the share class. So, if the float's 1 million shares, and 500 thousand are shorted, then there's 2 million shares held long. Let them all vote. Make this system a requirement for listing on a public exchange.

Whether a long is synthetic/borrowed or primary, their interests are aligned. They both want the company to do as well as possible. It also avoids voting rights shenanigans that exist, even with the current borrow-to-short system. At the very least it seems extending voting rights to synthetic longs won't ipso facto make corporate governance worse. I see no obvious downsides. And the academic evidence shows that removing constraints to short sales makes corporate governance significantly better.