I'm very interested in your solution.
I have a vehicle on order with Rambox and was looking for a way to carry bicycles over a tonneau cover with the cover closed (for road trips to keep luggage and such locked up). I liked the Retrax solution but they don't offer the Trax rail system with the Rambox. I'm also concerned about the fit interference issues of the Retrax as published in these forums.
My current thought is a BakFlip Revolver X4 with a hitch mount rack specifically the 1-UP USA. I was hoping to avoid a hitch rack but I haven't found any other solutions.
Edit: Just noticed Pace Edwards is in Centralia. I'm up in the Seattle area!
I will let you know what they come up with. I think what they are planning is a product similar to their Ultragroove but it will sit just inside and down from the edge of the Rambox. This will allow the Ramboxes to open and close without issue. Also they are helping me figure out some risers that will work with the T-Groove in the Ultragroove raising the bar high enough to allow the RamBox doors to open. This would allow for a bar that extends the entire bed width allowing more bikes to be loaded over the bed rails/Ramboxes. Should be interesting to see what they come up with.
I'm sure the couple hour drive down to Centralia would be well worth not paying shipping. Also if something eventually goes wrong with it your not far from the factory.