I think I found a likely solution. I bought a pile of these two rail trucks years ago and they have been sitting in the bottom of a box ever since. I don't even remember what brand they are.
It turns out the ends of the axle sets are the same diameter, 0.076", as the diameter of the lego axles. I think I will be able to snip the axle on a BMR wheel set and use the bearings on a new outside frame truck arrangement. A between the wheels bearing arrangement would still be better in opinion, but at 3 studs wide probably not that stable. The outside the wheels bearing arrangement will still likely be 7 wide.
I hate to waste a set of BMR wheels but this is for a prototype, so I might have to break a few eggs to get the omelet. Anyway I thought you guys would like to know what I found so far.