Some time ago, I wrote a library to allow the Brother PT-2450DX label printer to be controlled from C programs, without having to go through CUPS.
I haven't touched it in years, but the code is still available for perusal on a strictly as-is no-support basis.
Perhaps the most useful part of it is a tool called ptdecode, which reads P-Touch control codes from a file, decodes them, then displays the image data on the screen.
Mercurial repository: http://hg.philpem.me.uk/libptouch/