I know it's a late reply, but maybe you have not given up just yet.
I have been playing with exactly the same kind of print heads, and have print head from a Dymo Labelwriter 320 and from a portable Facsimile machine. Both use the same principle.
By digging around, i found out the manufacturer of the DYMO print head, and by writing them a mail, telling them a lie about i had a big amount of them i bought cheap, and i needed the datasheet to use them in a small production, they agreed and gave me the datasheet
Using these print heads are quite simple. You supply them with 5V logic, and when you want to print, you enable a 24V supply voltage, and start clocking in data in the shift registers - just like an 74HC595, just this one is a bit longer (1728 bits for the Facsimile unit) - you throw a latch and the data you just clocked in appears at the driver circuits, you send a strobe who pulses all the heating elements for a VERY brief period, you don't want to fry them.
According to the datasheet, the DYMO one draws 13,9 Amps when all 448 dots are pulsed, so you need to have a big cap across the 24V supply!!!
I have collected all i have regarding these printheads here: http://zapro.dk/div/printer/
I am in the process at looking at the signals closer with a logic analyser, and i am planning on writing some code for my Arduino to drive the units.