This blogger found one solution.
Make Portable Printing Easier with Print n Share and the Brother RJ-4040
Use an airprinter with a power inverter
see makethisstuffwork post in:
Brother PocketJet® 673
-- Prints on a variety of 8.5" wide thermal media.
-- Print PDF files from your iPhone and iPad with the PJ-673 Print app
"Mobi Print gives its users the capability to use iOS devices to accomplis iOS mobile printing, such as iPads, iPhones, and other Apple products, to print with any of Zebra’s mobile, CPL, & ZPL printers. "
select the iMZ320 Mobile Printer. Prints via an iPad app.
"Mobi Print gives its users the capability to print (bluetooth or wifi) PDF's, Images or Cloud based label formats to Zebra mobile or desktop printers on iOS devices."
Please be advised that Citizen provide MFi mobile printers which will communicate with your iPad or iPhone via Bluetooth. You can contact Citizen for further information here: http://www.citizen-europe.com/en/contact-us?prod_cat=Mobile&printer=CMP-30
HP has released the HP 1200w Mobile Print Accessory. This allows you to use an HP 100 or HP 150 mobile with your iPad or iPhone. Both of those printers run on battery so they are perfect for travel.
Here is the setup video: https://www.youtube.com/watch?v=9zVfCpT4X84
I downloaded an app, I'm at starbucks on a macbook air. It let me print to my hp network printer no problem. I'm sure you could use that and a USB printer. The app recognized my HP and drivers immediately. I'll respond again when I get home, giving you the name of the app. Might do a search on iTunes. Was very inexpensive and invaluable. A little convoluted in instructions, but once understood, very easy.
If you want to do this the most reliable way, forget airprint as that requires a network and wont work with mfi bluetooth printers.
Get Mobi Print and a mfi certified bluetooth printer like imz320 for receipts or qln series for labels.