You don't actually need a real mac serial. that's a misconception. you just need a serial not in use by someone else. you need to ensure that your hack stays with that ONE serial and you don't keep changing it. Every time you change system def in multibeast, serial changes, every time you change boot loaders it changes, unless you ensure you copy it from one install to a new one and put it back in there as same serial.
You need to make sure your rom and MLB are CONSISTENT in NVRAM. To do that you need that consistent serial, you need a consistent and working unchanging system ID. have all this, and imessage will work like a charm even without a real mac serial. If anything, if you pull a real mac serial (and that real mac is still in use) you are at greater risk of issues than the fake serial. Apple is cracking down lately on too many registrations and shared serials and thus all the "here is a code, call us" errors people are seeing now. too many serial changes or using a shared serial gets account flagged. I should know, my account was flagged, I found out I had over 10 registered serials from same hackintosh accumulated..Oops
Also, I tried a real mac serial at first, i got an error message with it, because the other mac was still signed in to imessage. If it's a retired mac, Then I suppose it's worth using. But don't go stealing your sisters/mothers/brothers, etc mac serial thinking you NEED it cause your fake doesn't work. Find out why your fake doesn't work (maybe you managed to generate a valid serial someone in china is using). Maybe you've registered too many serials and need to call apple and get your account reset.
Once you generate a fake serial, Check it online (plenty of info in the imessage guide thread on how to do this). Best serial actually is a unique bogus one that does NOT return as valid online (because valid means possibly in use somewhere).
I have working imessage and facetime on 10.10 with clover, with bogus serial.
http://mysticalos.com/10_10_Mesages.jpg
I've even enjoyed some handoff already and answered phone on my hack through facetime, forwarded from my iphone 5s. Follow Jay's imessage guide, its good.