The tubing is very slippery and combined with chain lube etc it is almost impossible for a zip tie to grip the tubing.
So when riding the chain is travelling backwards and rubs on the tubing , eventually taking the tube with it .
The tubing will NOT pass through a rear derailleur !
You idea will work , although tubing has the advantage of keep you clean and your clothing out of the chain if in the right place ?
I just sold one of these , you can see it actually just has 2 tubes and no chain rollers at all !
Bit of pedal drag but simple and quite.
By the way have you put some sort of keeper on your BMX thingie to stop the chain jumping off ?
Idlers the bit of metal shown here.