I assume this is a hot water system, not steam?
Even still, as much as I like pex, i wouldn't want to see pex used for a radiator system. You're flirting with its temperature limits. Its good for radiant floors (which usually are operating more in the 125 degree range), but I'd be wary of using it for radiators, which usually operate in the 160-180 degree range (180 is usually the max rating for the piping). I think Uponor might actually have a max 200 degree rating (if memory serves me right), which would ease the concern a bit.
That's not to say that it can't be done, but you better be careful with your controls and run a conservative temperature, which might require higher pump speed to provide adequate heat.
Is there no access to make a transition from the pex to a brass stubout? You might be best off making a hole in the ceiling below if necessary to make the change.
I think the O.D. of 1" is 1 1/8". You could probably slide a 1 1/4" pipe over top of the exposed pex, make the pex connection, then slide the pipe back up and hold it in place with an escutcheon.