Often people maintaining courts will just take the cranks used to tighten the cable away and keep them so people don't mess with the nets - so you might be able to get yourself a tennis net crank and carry it with you... you should be able to get one for $30 or $40... but if they lock the mechanisms at the public courts like I've seen in some cases, this may be of little use to you...At my local public courts, the PBers slant the net strap to lower the net, then when they leave they don't put it back. This has had the effect of lowering the tension of the net and even if I move the net strap back to its proper placement (straight up and down), the net will no longer spring back up, and so the net strap just hangs there loosely. Unfortunately, at these public courts there is no easy way to adjust the net tension - though I'm going to look into tools/options.
For those who play PB, would it really be that big of a deal if the net height were the same for both tennis and PB? And if you do play PB, please at least straighten the net strap when you're done.