This is not just an issue of adding a cool feature, but a safety issue as well. One way trails are usually high speed or too narrow for passing. It would be nice to be able to see at a glance what trails are available to ride without getting to the wrong end and having to replan the ride on the go. One-way trail options could also solve an earlier issue of wanting to show summer ski lifts to aid in route planning.
This can be as simple a fix as adding a new line type consisting of small arrow heads in lieu of dashes.