According to http://safetravel.dot.gov/larger_batt.html
, you're allowed one "larger battery" on carry-on 'installed on a device' and upto two extra 'larger batteries' in luggage. The max size for a larger battery is 300 Wh.
I'd imagine something that looks custom made would buy you more scrutiny, but it seems like it'd be okay. Personally, I'd just put the batteries in some kind of case with the appropriate rating and put it in luggage and leave the battery off the scooter itself. That would seem to have minimal hassle and risk. I don't know what kind of case I'd put lipo in, though, to make it "official looking".
Apparently, you can have an unlimited amount of "smaller batteries" (Less than somewhere around 100 watthours), so maybe you could just bring whatever amount of hobbycity lipo bricks that you wanted in its raw form assuming a rating of less than 100 watthours. I'd think the bare wires would probably be suspicious, though.
This is probably where strap-on dewalt packs would probably be best. It looks like the normal dewalt packs are ~70 watthours or so, so you could take on as many as you wanted since it's not a "larger battery" according to 'their' specs.
Btw, I feel kind of stupid. It looks like the original poster disappeared (I never saw the original poster)?