I’ve been using Ctrl-s for years. Nothing else uses it because it’s historically been used for terminal flow control, but that doesn’t really have much use in a graphical terminal with history and scrollbars and so I’ve never missed it. Has similar two-finger ergonomics to the Ctrl-a bind as well.

edit: oh, and I think Ctrl-space is the keystroke to get a nul character, and vi uses that character to insert the last entered piece of text in insert mode. It looks like Ctrl-@ does the same things, but tmux might not be able to tell the difference either.