I recently updated and revamped my old paper texture script into an ok parchment texture script, and thought you guys might have use for it.
Example:
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parchmenttexturebynaeddyr.scm.zip

The script uses the currently open image as input, duplicates, flattens and then starts working on it. There are several output layers and some layer masks (flattening is toggleable in the script), so it takes a while to finish the process, especially for bigger images. The default opacity settings of all the layers default to "err on the visible side", so lowering the opacity might be the most common task users should do after getting the output image. The other is playing with the image masks; the plasma layers are fun to play around with using Blur, then leveling, and the round blend mask of the edge grunge layer should be fixed if the image is not particularly square (or disable, if the image is large).

The script is basically a version of my general parchment/paper texture methods, except of course not as good because it's all mechanically random. The output isn't really that bad for a script, though, so, hope you enjoy!