For anyone who uses timidity, the trick is to use the -K option, e.g: timidity -iA -B2,8 -Os -K 2
From the man page: "-K n, --adjust-key=n; Adjusts key (i.e., transposes the song) by n half tones. Ranges from -24 to 24."
Hope it helps someone!
PS: and of course, you need to connect the keyboard with e.g. aconnect 24:0 128:0 (assuming you get 24 from aconnect -l)