I agree with Michele. You can use plastisols, they will look like it was plastisol screen printed in, except you apply them yourself with your heat press, there are some nice vinyls out there (but then you have to cut and weed them), not too sure on the inkjet papers though since your shirts are dark, those papers have a white background.
Here is a link to the section of the forum that deals with all things relabeling:
T-Shirt Tag Relabeling and Finishing - T-Shirt Forums
Bet you'll find some great threads there. Good luck. =)