I tried p9p Acme on Mac OS X, seems the mouse button emulation with trackpad is broken, p9p Acme doesn't response to Shift + Secondary Click, this is probably my problem, won't play it now until I'm more familiar with 9front in Virtualbox.

Alt+Click does a middle click (button 2), Command+Click does a right click (button 3). Also, Command+Z does what's expected.