I get where you're going with the channels. Makes perfect sense from a developers perspective given that in game chat is coded in house, while party chat is likely a call to a pre-developed module provided by xbl's infrastructure. Getting them to play nice together may have been more trouble than it was worth... but it's been done in the past.
With that said, how sure are you that this wasn't supported in other versions of COD? Cause I'm pretty sure it was supported in either BO2 or MW3, possibly both. This isn't a feature I made up in my head, it's something I miss based on having it beforehand. Honest!
Now you have me doubting myself and I'll have to check tonight.