I suppose if it was "easiest" or "most convenient" to do the offline transpositions on a 51 development platform then one deals with the source code filing onto the PC somehow. Did you squirt it back to the PC through a Serial Port?
we have a program made by out european subsidary that makes the "image files" (no more to say - proprietary) which previously was preprocessed by a US purchased piece of PC software. The support for the purchased software has been lacking (especially when we make a change in the "image maker") so it has been decided to not require the use of the purchased software and rather than making another converter we accept either format. Now one is horizontal font and one is vertical font and the differences are such that a 'preprocessor' that basically does what the purchased software did if needed (many signs run on the old format and our customers would not appreciate a decree "convert or die").

Erik

PS our images do not come from a PC, but a USB stick written on a PC