... and I can't turn off the water supply line?

Along one side of my property runs a buried irrigation line which remains turned on from about April thru September. At intervals of about 100-200 feet are 1" pvc pipes coming straight up from the main line so that people can access the water. Each of these is supposed to have a shut-off valve right at ground level, but mine was apparently broken off (apparently by accident) just below this point and capped off by the previous owners of this place. I suspect this happened during the summer months when the water was turned on or else the previous owner might have gone to the trouble of adding another shut-off valve instead of capping it off (I wish he had!)

Anyways now I need access to this water and there's no way I can turn off the supply line. Ideally one would wait until they shut it off in the fall to work on it. But what happens if I cut the pipe and try to glue a new shut-off valve onto it with the water turned on? I can prime both sides of the fitting but I'll probably only be able to apply glue to the shut-off valve because the other side will get too wet as soon as I saw the end off - and really the glue on the shut-off valve will get wet too but at least not until after I get the glue on first. Then I can stick it together and set a big rock on top of it or something to hold it on there. The big question is will the glue hold without water seeping through the fitting and ruining it? I've never tried this before. I'm guessing it will be best to leave the shut-off valve open to reduce pressure on the fitting/glue. But then everything will get wet. Anyways, I know this is generally a dumb idea but if I really need to do it, is it possibly feasible?