It depends on your testing methods, but also remember that casinos must monitor the RTP of games and report games that run under target to the UKGC. Of course for individual players RTP can be wildly off, but en masse it shouldn't be...
So a game running at 70% for the first 300 games should, in theory, trigger that. Because everyone would get that RTP. And you would then need a lot of people to carry on playing to get that back up.
And even if it wasn't strictly illegal, it would absolutely draw the ire of the UKGC. I hope the people you know don't do it, although I undertand the reasons for considering it.
I know WH, in retail, ask for RTP to be proved not only over long sessions but also over short ones. For example, you run 1 session of 10m games and check the RTP. Then you run 1m sessions of 10 games. The RTP should be the same... but in your example it would be miles off.
I don't know if online providers do this, but the moment one company gets caught doing what you are saying (and either UKGC, GLI or the more reputable casinos may well find this and report it), some people are getting huge fines.
There is no way that is "within the spirit of the Act"