I have tried a single 626R as a center speaker, and a pair of them, horizontally head to head on top of a 65" Mitsubishi RPTV. The pair sounded substantially better than the single to me. Richer midrange and more intelligible dialog. Based on this I have ordered an RM-30C to use as a center channel.
Even though this will not be a completely symmetrical center speaker, I think the advantages of the improved midrange clarity will outweigh any downside. It should also blend better with RM-30 or 40 front speakers.
As for a problem with the weight, two 626R's have a combined weight of around 90 lbs, which, when spread over their side surface areas, did not seem to be any problem at all. The RM-30C should weigh about 110 lbs, but will have a larger side surface area, since it is deeper by about 6". I am planning on using 1/2" grey foam under the whole speaker to cushion it, and additional smaller pieces across the back to have it angle down slightly aimed at the primary listening position.
The real test will have to await the arrival of the speakers. Stay tuned.