It is an interesting plan. Before you go through with it, I think you should double-check that all of your equipment really requires a converter and not an adapter, because most modern devices can take either 110/120 or 220/240 as input without any problems. I did fry a battery charger by plugging it into 220 because I thought it could accept that voltage. I read that it was 110 only after the fact.

As far as putting a UPS in luggage, I believe it is a DOT requirement that you unplug the battery for shipment. If you notice, all new UPSs come charged and the battery comes unplugged. I have shipped UPSs for work by both ground and air, and they certainly get shipped by air delivery companies, but always with the battery unplugged. From a safety standpoint I don't think there's any issue with it. You'll have to double-check with NWA if they have any sort of a regulation that would prohibit you from packing it in your luggage.

BTW, aren't you concerned about the extra weight?