i dont see why SSH wouldnt work for you. that is what i use. when establishing the ssh connection i setup port forwards so that i can connect using rdp to XP/2003 for management. then when i want to remote desktop to an xp or 2003 OS i use rdesktop and point it to my port foward for whatever machine i want to connect to.
ssh -l username-on-linux -L9000:ip1:3389 -L9001:ip2:3389 -L9002:ip3:3389 <and so on> linux.ip.or.domain.name
or you may look into setting up a VPN which may be more suited for you
if you are using a windows machine to connect RDP then use putty and you can specify all your forwards in putty's configuration and save it.
if using XP machine to connect to your linux gateway you will need to copy the mstsc files to a diff dir and set 98 compat mode for mstsc.exe, then you would use the 98 compat mode mstsc to connect to localhost:900x and it will forward to the machine behind the firewall through your ssh connection
hope this helps...