can I charge or provide power via a 28 volt dc system as in a cessna 182

Does it have a cigar lighter receptacle? Look for a USB charging adapter with an input rated 12 - 28 Vdc. Its output must be regulated to 5 V for the iPad.

Here's one: http://www.amazon.com/Charging-Adapter-Aircraft-Cockpits-12-30VDC/dp/B007672CYC

You can get AC 110 V and DC 15 V systems for DC 28 V in light aircraft. Won't be cheap.

Truckers converter could convert DC 28 V down to DC 14 or 15 V if adjusted properly, but may not be adequately cooled at altitude.

Power Macs ran on DC 24 V (without surges) Intel Mac MagSafe now much lower. Both run off clean DC 15 V with airline adaptor.