I'm from a BBIB (though admittedly real estate) so will give you my thoughts about equity research.
At major Ibanks, I can't imagine its that easy to get into. On the one hand an MBA would prepare you well to do the "actual job", learning how do value companies, doing PE ratios and all that jazz. On the other hand.......equity research teams tend to be small and you will have a lot of competition for the open spots.
I think the bottom line is......equity research is not easy to get into.
The CFA is actually probably quite useful for equity research, a fair proportion of research analysts have them or are in the process of taking them. So if you can get the level 3 done that would be a positive.
An MBA is still a better route in though. When you write your essays about wanting to do equity research you really need to craft a coherent story about which particular sector you want to cover and how your previous experiences made u realise this........and how an MBA will enable you to make the transition. Having done CDO Trust Services........Im not sure how this story would go (!) but Im sure if you ask yourself why you want to do ER/IM you can craft a compelling story.