I have a similar issue. NSR light bulb is receiving no current (consequently doesnt light up), no warning light however. If I remove the bulb then the warning light flashes up "left rear side light failure", if I then put it back in the warning goes off and the side light comes on!! As soon as you turn either the ignition or lights off and on again you are back to no current and no light! The OSR and numberplate lights are working fine, so I too was thinking about running a positive feed from the OSR cluster, but like you I am concerned this may duff something else up. Blasted elecronic computers, I think the body computers are c£400?
Any suggestions welcome, this sounds like a fairly common issue from what I have seen on the net, but no real ways of testing or resolving.