Are you proposing a common spelling of "musta" ("woulda", etc)?
I only find "must've" weird as it's not a very commonly seen contraction. Much like "mustn't". They're definitely used in speech, but people rarely type them out, which is how you end up with people using "of" when they mean "have" or "'ve".
I don't think I've come across anyone using "must" and "not" in a contractional form. I shudder to think what the "of" crowd would type for "mustn't".
