Having never tried it, or even thought about it, is it possible to send a text from a cell phone from a aircraft?
As demonstrated by UA93 and several others on this forum, yes. However typically only from below FL250, and in areas of low cell density.
I think it would be quite improper for RR or Boeing or the Comms vendor to make a statement...
Under normal circumstances yes, but in current situation it is quite unusual that RR does not confirm the categorical statement made by the Malaysian authorities. I this case the source of data is RR, so if the Malaysians make any statement based on this data, I see little reason why RR would not want to second it to dispel any lingering doubts.