As far as I know, BT -10 mylar tube is not available. It was used as an ultra-light tube for contest purposes. But glue would not stick to it, so you had to wrap it with a paper sticker anyway. I am not convinced that this was any weight advantage over regular BT-20. That is what I used on my clone. It worked just like every other one of the Streaks that I built as a kid:
This was on a 1/2A6-2!! Oh well, I am sure I will build and lose a few more over the next few years!!
Your substitution of BT-20 is probably a much better idea, especially if you are going to paint the rocket anyway (why kill yourself to find see-through plastic tubing and then paint it?)
I built about three of the little Astron Streak rockets when I was a kid, and I remember that plastic BT as being extremely wimpy. It was very crushable, and did not have anywhere near the strength of the regular paper BTs. The only reason to use the mylar for BT material was to have a see-through rocket, but as astronboy has already noted, you had to use a sticky-paper applique on the nose and tail to be able to glue on the NC and fins, so there wasn't much of the BT left to actually 'see' through.
And after you flew it a few times and the ejection charge scorched the insides, you couldn't see through what was left!