I'm kind of digging the idea. Rather than fermenting with the sweetened condensed milk, maybe blend it in the glass. Kind of a beer version of a White Russian. Or a beer float.
You can actually ferment milk, but it helps if you start with that unnatural stuff that's made for lactose intolerant people. The process creates cheese curds from the fats and proteins, which needs to be separated from the liquor. However, brettanomyces can ferment lactose (I think most strains of brux can, but don't quote me on that - Edit - make that anomalus, not brux).
If I were going to attempt this, I'd probably ferment the milk separately with either brewers yeast and lactose-free milk, or throw caution to the wind and go all brett. Brew the stout separately, and blend them in secondary. Give it a good 6 months to fully attenuate and see how putrid it gets before you toss it.
Now I kind of want to try this...