session-local viminfo file

How about a session-local viminfo file: This can be achieved by the autocmd

autocmd vimrc SessionLoadPost *
      \ let g:local_viminfo = findfile('.viminfo','.;')|
      \ if !empty(g:local_viminfo) | substitute(&viminfo, ',n.*$', '', '') | let &viminfo .= ",n" . g:local_viminfo | rviminfo! | endif

and calling :wviminfo after :mksession.