AVR-Chat

>
>the printer mechanism and controller both operate from 5V. I am not 
>intending to directly drive the logic, but to interface to the 
>printer controller by RS232.


Ok, I understand.

You can probably find something at your local model airplane shop, in Lithium Polymer batteries, that will fit. They aren't cheap, but they offer huge output power, and you get >7V with two cells.  Then you just need a buck reg for your logic.  Your printhead should be able to operate here. 

The primary determinant here will be the printhead's operating range. 
Look for the "do not exceed" numbers in the data sheet.