I looked at that (like I looked at all avenues even though I know I want to go to the airlines) as a possibility especially if my military time + theoretical CBP Federal service time = federal retirement after 20yrs or something ~20. Unfortunately I can't remember what my research concluded on that.
However, I also reminded myself that the main reason I plan on getting out is that I no longer want Uncle Same telling me which hand to wipe with...so I crossed CBP off the list.
I don't have any particular insight, but I doubt that too many CBP agents will leave for the airlines unless they are young and happen to be at the end of their federal commitment. I know a lot of their hires are prior mil who clearly decided that CBP was the route they wanted to take.
I did follow their hiring windows for 2-3 years and in short, there weren't many of them. So unless you are willing to flight instruct or something while waiting for a hiring window to open..it'll take a fair bit of luck to get hired on right off of AD.
I know that was a lot of words that don't really answer your questions...just my .02 on CBP. It really is a decent option if you want the 'stability' of a federal job, though.
Let's be real, you just want the 'Federal Agent' badge for Friday nights at the bar...am I right??