I've tried it with relatively recent versions of x11vnc and
tightvncserver on Debian Jessie and Linux Mint. It works. Apparently, it
doesn't work with older versions of various VNC servers on various
operating systems - upon closer inspection, it seems that the Alt key
tends to get "stuck" if you release it before releasing Shift... but I
don't think this is something that really needs to be fixed.
I think we can close this bug now. Sorry for not checking it with recent
versions of VNC servers in the beginning - I did not have an opportunity
back then...
--
darkpenguin