Port forwarding alone is insufficient… at least since about a month after iTunes was released. Due to belly-aching by the RIAA, Apple made it a requirement that DAAP clients be on the same subnet in order to “see” a share. That said, I do not know (since it is the actual client in this instance) whether the SoundBridge has the same limiations. Regardless, port forwarding alone is not going to relay your mt-daapd service at work to your home LAN. You are still going to need a beacon relay on your home subnet and I am not sure one can do so by installing mDNSPosix on the router — or even if the latter is possible.
Interesting idea: creating the SSH tunnel to the router. It should work, but, again, I think is insufficient in and of itself. The beacon is the integral part. A relay must exist on your subnet to rebroadcast the share made available by the remote mt-daapd.
WRT the command in the wiki, that is both obsolete and partially incorrect. It is obsolete, because the -R switch is no longer required. The error derives from a point Ron brought to my attention in this thread. Apple’s mDNS implementation does not support proxy beacons. However, Apple still maintains the mDNSPosix package, which does contain mDNSProxyResponderPosix. That command should be something along the lines of:
mDNSProxyResponderPosix <serverIP> squeal “shareName” _daap._tcp. 3689 &
You could experiment with that command and substitute the IP address of your router or, perhaps, that of your mt-daapd machine at work for the serverIP, but I have never tried the latter. Using the router address should work; providing you can successfully tunnel port 3689 from your work machine. However, I think you will still need to initiate it from a machine on your home LAN.