I'm not speaking specifically about FedEx...
I can't think of an example where any airline ever paid an overtime bonus (125 or 150%) for work voluntarily picked up.
You have to be careful about what you wish for....
Unless a strict and enforceable staffing model were coupled with an overtime bonus, companies would love to pay a bonus for uncovered trips, and pilots would be more than willing to pick up the work! The bean counters know EXACTLY where the break even point of overtime vs. hiring is located. At UPS we attempted to establish the concept of "overtime" in our mid-term TA that was turned down over five years ago. In that case any portion of a line built >75 hours would be paid at 125%, a sort of "soft cap."
In the TA we just approved, we are finally getting retirement benefits PAID on any overtime work we pick up or are JA'd (drafted) for on a day off.That used to be a huge cost savings for the company: pay a guy at a straight rate with no additional benefits in lieu of hiring a new body.
What IS odd is that at UPS if your awarded bid line is worth 84 hours, you still can't legally dump it back down to 75 hours, so we are in effect still working "overtime" at the straight rate.