While this doesn't exactly help you with delta, I will confirm that:

* As a gold member, AA has let me borrow miles in the past w/o any fuss.

* Even though as an elite (albeit a lowly silver), CO absolutely refused to let me go negative for <1000mi. I ended up having to buy those last miles for a stupid $32 bucks even though I already had (3) IAH-LAX flights "on the books and paid up" for the next month...

Yet another reason why Houston sucks.
-g