Assuming there are characters in a demo file and you like one of the costumes, can that portion of the demo file be converted to a costume file?
What Taqh said about the validation when using the tailor.
But it probably wouldn't be too difficult to parse the text, automatically or manually, into a *.costume file. Just copy the format of existing costume files and insert the different data for each costume part. The colours are in different formats: Demos use hex values while COSTUMES use decimal, but just convert the RGB (red/green/blue) hex values to decimal. Until you learn the format, this might be trial and error, but it's really not too difficult to learn the format.
After that, when you visit the tailor in PChat and load the file, it will tell you whether or not the costume validates. It'll say "cannot find the costume part" for this body part or somesuch, with an option to "fix" the part (fixing will screw up the intended costume in PChat, but won't harm the *.costume file).
For those that don't want to listen to the PChat tailor comes the great world of database "hacking."