Wish I had a good answer for this one. The one thing about cutting tubes is at least if the blade is doing the primary cut going into the tube the then that outside surface will be clean. That is why you get a cleaner, square cut when you spin the tube either on the fence of a radial arm saw or chop box or crosswise on a table saw. No matter what though even with a super duper blade you will have some fuzz that needs to be cleaned up.
Without a table saw or chop box I would be tempted to clamp four 4-wheel dollies upside down on a work table so that they can be placed in such a way as to be able to spin the tube maintaining a consistent angle. That way you would at least be guaranteed a square cut and, if spinning the tube into the cut, get minimal blow out using your circular saw. Sounds like something I might try if I was doing multiple tubes but not worth it for one I don't think. For one offs I have just wrapped tape (super low tack) around the tubes and hand cut with whatever (jigsaw, handsaw, etc...) and then had to spend a bunch of time cleaning it up. Good luck! Please let us know what you come up with!
Cheers
Ashley