Tried it today with an actual flag pole.  That didn't work.  The sections of the flagpole I tried had an ID of nearly 40mm (1.5in).  The extra diameter caused the hose to buckle and fold back on itself instead of sliding inside the tube.

Then I tried it with a 7.3m (24ft) 3 section painting pole like this one:

The center section diameter was too small to fit the hose inside, so I removed it.  The remaining section then fully telescoped under about 40psi, and then retracted when the pressure was removed with no issues!  Well one issue in that I removed the pressure too quickly so it came plummeting down and nearly cut the hose off..

That's only a 2:1 increase in length cause I had to remove the center to do it, but the basic idea here works.  Using this pole won't actually work though, as I'm planning to spin the body of the pole for rotating the antenna.  The smooth sections without the locking collars engaged will spin freely, and so my antenna would spin freely..

This one though, is ribbed so I can use those to keep the pole from rotating.