Eu encontrei uma solução alternativa.
Vários servidores com a opção -L
(que especifica o nome do soquete).
# set variable
# create 2 sessions
$ export VAR=aaaaa
$ tmux -L $VAR new -A -s $VAR -d
$ export VAR=bbbbb
$ tmux -L $VAR new -A -s $VAR -d
$ tmux ls
failed to connect to server
$ tmux -L aaaaa ls
aaaaa: 1 windows (created Fri Jan 23 12:12:46 2015) [207x50]
$ tmux -L bbbbb ls
bbbbb: 1 windows (created Fri Jan 23 12:12:55 2015) [207x50]
# connect to sessions
# check value of variable
$ tmux -L aaaaa att -t aaaaa
$ echo $VAR
aaaaa
$ tmux -L bbbbb att -t bbbbb
$ echo $VAR
bbbbb <-------- now i can see 'bbbbb' here