not exactly what you want but ...I've had great success withwaypoints(marks) and the "send to GPS" with an old Garmin12xl on the serial
port. The waypoints went over while the gps was in use
which I thought was awfully convenenient! then ,undocked, it's just a matter of using the GOTO button per each wpt...
But I know that's not exactly your question...
It might take less time to "Export Route" to a memory card and plug
it into a newish gps that can read GPX format as it might to "Send To GPS"..than if you would have to change the GPS settings to get it to Accept stuff through the "Port", if you see what I mean.
If you happen to already own a Troublesome handheld, (the Troublesome Co. has such a large market share!), GPSBabel (Free!) will be your friend to convert the exported GPX route to a peculiar proprietary format and upload to the handheld via memory card ,usb or serial
But this can still be very fast.