I'm a moderator for Rainbow Six Vegas and we have one category, All Terrorist Hunts, that gives in game times that we'd like to use. Then there's story mode which does not give an in game time, but we'd like to set this to time without loads by default. I don't see a way to do one for one category and one for the other, I only see how to change it for the entire game (all categories).

Am I missing something or is what I'm describing impossible?

No. There is no way to have the default based in category afaik. We struggled with this in cuphead for a while, where we eventually switched to only one timing method for all categories as the distinction doesn't really matter too much (at least for our community). If you only have the default time field, it comes up as Time, and there's only one field, so it works well enough