I don't know of any plans to modify the bass management to allow large mains in stereo and small mains in surround (the arrangement which you are interested in).

If you connected the 2910's stereo analog output to a spare stereo input on the 990 and set that input to stereo bypass you would get a full range signal to your mains, but since your mains are set to small you would also get a signal to your sub - not really a solution in your case.