Just turn the short "o" into a string of "a". I wouldn't try to add on any loaded allusions to Mars or cows going moo, or else the reader will suppose that there's some such allusion in the Russian. It's enough to make clear that it's some sort of habitual broadening of the vowel.


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Note added at 2 days16 hrs (2015-05-28 17:40:29 GMT)
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Misha, yes, I know that. I was taught Russian pronunciation by a княгиня who had graduated from Smolny and held that "когда я умру, русский язык со мною исчезнет" (at least that's how I remember it; perhaps her wording was more exquisite), and when I subsequently arrived in Leningrad to attend Russian-language courses for foreign students, one of the instructors was from Moscow, and I couldn't understand anything she said. Not only because of аканье, but because she seemed to speak without opening her mouth. All I'm saying is that even if it has a particular funny sound to non-Muscovites, I think it would be better not to load down the attempt to render that with some oddball allusions, or to make it sound like some particular regional accent in English.

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Note added at 4 days (2015-05-30 20:30:28 GMT) Post-grading
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Oh, - I didn't write it, but never intended to propose not writing "-cow" at the end of the word, since that's how the name of the city is written in English already. Or, do you mean you're going to split it off as as a separate word, something like "Ma-a-a-a-s cow", versus "Ma-a-a-a-scow"? Just wondering.