Hi jmducat,
It sounds like what you really need is to be able to assign a special menu to the RSM, rather than just re-use the existing menu (since each menu item can only have one URL). That's not something that can be done currently, but I do hope to add that functionality somewhere down the line.
For now, your best option might be to use some custom JS to reroute the item manually. Or, if you control the external site, do some browser detection and route the user to the mobile site on that end instead.
Best,
Chris