I've got a TraceR which I mount on the seat tube and it shines out between the bottom of the saddlebag and over the top of the tyre. Depending on frame size, this may work for you?
You're getting a custom frame right? Get an hourglass eyelet or something popped on the DS seatstay, should be able to bolt/bodge any number of lights on (Cateye holy hand grenade being my fav option for this)
What rear lights can mount nicely to the stays rather than the seatpost? I'll have a seatpack on which means seatpost is a no-go.
I like the look of the exposure TraceR but not sure if the mount is compatible. Don't want to bodge it on - a properly secure mount would be nice.