Hi Matthew,
I would love to see Philips add a ton more properties and methods into Prontoscript from the generic javascript, but from what I've seen it's not likely in the near future. It's not a bad thing; you just have to use a different path. I have yet to be stopped by a lack of functionality in Prontoscript although I could do a lot of things a lot quicker if there was more functionality.

All that being said: I'm not aware of a property that would allow you to adjust IR duration during runtime. I think you've already found the solution by the looks of it. Creating two instances of the same IR function; one with an extended duration and the other at default would definitely address the issue.

Cheers // Jason