Thanks Stuart. We will be adding this feature. Please be aware though that it is subject to some limitations, most notable of which is that the directions saved with your route on plotaroute can't be transferred using Garmin's API - instead, Garmin will attempt to generate turn instructions before sending the route to your device.
Great news! Looking forward to it - shout if you want any technically-savvy beta testers on board :-)
FWIW, different devices handle turn-by-turn differently, but the newer ones (e.g. my own Edge 830) generates them on the fly on the device itself, from an arbitrary GPX when you first start the course. This seems to work well and I suspect Garmin's own interface will decide whether or not a given device needs server-side turn instructions added before being sent.
Thanks Stuart, that's good to hear your positive experience of getting turn instructions from a GPX on a Garmin. And thanks for the offer of help with testing - we may well take you up on that.