[Stooky Bill]

With clever maths it is possible to remove a service from a statmuxed
Stream and substitute another service which  itself can be in a satmuxed stream.
All without decoding  the substituting service …..
I demonstrated this to the BBC on 6/7/2005 ….You could not really see the join !

That was a big problem in the early days of DTT too, hence BBC1 was at a fixed rate as it was easier to replace in each region than if it was variable bit rate. Now that's not how it's done so isn't an issue any more. 

I'm pretty sure it's possible to achieve what you demonstrated quite easily these days,its quite a standard function in amy multiplexer hardware isn't it?