I think I can get something working for you.
First a list of questions for you to fill out.
1.) What speed and protocol for the serial interface? (if you don't have a preference I would recommend 9600 baud 8N1. Somewhat standard and slow enough that most any device can do it)
2.) What voltage levels do you want to run on the interface? (3.3V ttl, 5V ttl, true RS232 levels (+/-12V), something else?)
3.) Do you have any PIC programing capabilities? (If not I can supply programmed parts in small quantities.)
4.) do you want a fully assembled solution (serial + power in one end IR out the other) or just design data (ie buy parts and build on your own)
5.) Where are you located at (Do you happen to be near Rochester, MN? Else can ship things just adds more cost)
If you allow me to post the design of the converter part to my website and your using it for non-commercial use and WYSIWYG (no guarantee on usability or suitability) I would be willing to do the design part for a printer and a roll of paper (if you have a spare else open to what ever). Any prototype hardware you need me to build would be at parts cost + shipping. (if your local pickup is free) All other cases let me know what your plan is and we can negotiate
If that sounds good will do some digesting of the spec this weekend and come back with a updated plan (possibly code and hardware too depending on how things go). Once I get the initial stuff working will need a printer to verify it and tune the IR timings.
Let me know what you think.