Funny thought. I have a 2007 25' Intnl in my shop right now and I was thinking of the same thing! I've only seen the square front window on ebay and the forums classifieds. That doesn't mean that it isn't available from Airstream though. Airstream has or can make a lot of the older parts.
As far as adapting a newer frameless window to an older trailer like your '69-I think that can be done with a few small mods. Having seen what you can do with an old Armstrong, I'll bet it would be easy for you.
Work is never done, so take time to play!