[Led]

Why not just mix with realtime loudness metering? I'm with Nathaniel, it's so easy to tweek things a little here and there to comply when you are following the loudness in realtime, then your stems will sum to the right target. I find working without realtime metering a bit like driving with the windscreen covered up after getting used to it.

Although the nerd in me thinks it's an interesting mathematical question, I mean summing is just numbers in theory right..? Provided you are doing all your processing on the stem masters when you run out the full mix,
and not doing any sort of 'master bus' processing, wouldn't it just be a matter of applying the same gain (in LU) to each stem that Nugen applies to your stereo mix to get it to target? You can just analyze each stem in Nugen, then set the target to equal the stem lkfs + the amount of gain Nugen applied to your stereo mix then process.
I'll have to try it tomorrow.