One part is that you need a USB host implementation, this is much more complex to do than the device. But I guess you know all about this already...
The second problem would be to generate the pages. Old printers would accept asci strings as input and print it as text but almost all modern printer is of GDI type and then the printed data is generated and rendered by the pc driver. The format of the data sent is printer specific but basicly its 4 bit planes with CMYK data compressed. One popular compression used is JBIG.
For a more simple solution maybe a postscript or PCL printer could be used, then you would be able to send text strings as the printer is able to render informaiton by itself.