I don't understand what it is you are advocating here if you want to tether an RC plane to a stake on the ground - or even to a handle in your hand - then go ahead and do it. There is no "law" or rule against it, for sport flying. Frankly, this subject comes up more often than you'd think, and the general consensus is "sure go ahead", but what no one has explained yet is what advantage you gain. It takes away the part of control line that makes it control line - the direct control of the model through the lines - and it takes away that part of RC that is its advantage - the ability to fly the model using all the control axes anywhere it the sky that you want to fly it. And as for keeping it in a smallish area, you don't need a tether for that, you just need to turn before you get to the end of the field. They are called Park Flyers.

As it happens, I HAVE done what you suggest - as a demo for some youth group that was held in a confined space. We rigged a Sig Kadet with a tether cable (probably 65 or 70 feet) and the other end was hooked to an EZ-Just handle. One guy (me, as it worked out) held the handle, and another guy gave each of the kids a take-off, a few laps of elevator-wiggling, and a landing. It served its purpose in that in a relatively short amount of time we gave several dozen kids a little stick time and a brief exposure to what models are, but we all agreed that the flying was totally lacking in any fun whatsoever. As I said, it just took too much of what constitutes the "FUN" from both the C/L or the R/C experience.

As for the frequency control or the channel selection, I don't see where merely tethering an RC model makes in NOT an RC model, so I don't see where any different frequencies or extra frequency control is necessary. You still need to co-ordinate with any possible other RC activity that might be near or risk being shot down or shooting someone else down.