Think about adding an automatic air venting valve.

If you can add this to the high spot of the section that can trap air,
it should do the trick with no problem. They can dribble a few drops
now and then when it is venting, but you could put it under a sink or
somewhere else safe.

Not possible, I'm afraid. I don't know if there are any local maxima in the
line, but if they are, they are in inaccessible locations because ceilings
have been sheet-rocked already, etc.

Also, think about whether or not moving the check valve to the other
end of the line it's in would help the situation. It shouldn't matter
where in the line it's located as long as it faces the correct
direction.

I can't see how moving the check valve would help -- and anyway, there's
only one place where it will be readily accessible.

Air shouldn't be trapped in this line in the first place, under normal
circumstances -- it's only if the line needs to be drained for some reason
that air can get in.