I remember a discussion on reddit saying there was a US dialect (perhaps PNW?) that changed the pronunciation of the -man/-men part of the word rather than the o, but I couldn't get many further details at the time.
As someone who learned English as a second language. Yes, that pronunciation exists, I've heard it used on films. I don't know if it is a formally defined or linguistically studied thing. But I can hear the different ways the exact same word is vocalized wildly different by different native English speakers. And they always claim theirs is the only correct way of saying it, even though they still somehow understood what was said.
We do also tend to change the o at the same time, at least I do. Although I spent 10 years in the uk in my 20's so that has had some effect on how I speak.