can you program the rate button to activate AEB like a 1D does with its button press?
I'd say yes, the RATE button can be intercepted by ML (see /platform/5D3.113/gui.h) and any other possible function can be performed. This includes simulating other keys or directly accessing Canon DryOS functions that have been discovered. Activating Canon AEB (and setting it to different spacings/settings on consecutive keypresses if you want it) should be among them.
If you want this implemented and cannot hack together a module yourself after looking at the sample code, do a detailed feature request in the ML forum and explain why this is important to a broader userbase. It should be pretty easy to do, I changed several buttons on my personal 6d ML build. ML is nearly feature-overloaded as it is so adding yet another one might need some persuasion: your request is rather specific and the ML guys prefer their own bracketing over Canon's version.
ML badly needs a button manager for these things, but it's a lot of gui work so no one has had the enthusiasm yet. You also might get the reply that this isn't something to be added in C code, but by user scripting - but afaik this is wip and doesn't work atm (yeah, 3 internet geek abbreviations in one sentence :->).