By request at GimpTalk...
A script to take the current pattern, scale it and leave a copy in the clipboard. Gimp 2.6 lets you use the image in the clipboard as a fill pattern, so you can just use it from there
Scale is from 20% to 500% It used Lanczos scaling working a a tiled copy of the image, so if the source pattern is tileable the scaled one will be as well.
You can access the script by right clicking on a pattern and selecting "Scale Pattern..."
I have added an option to call either sharpen or unsharp mask interactively so the sharpness can be adjusted visually before the pattern is placed in the clipboard.
Updated to v1.2 - Now handles patterns rescaled larger than 512x512. If larger than 512x512 it will save the scaled pattern to a file, refresh patterns, and set the active pattern to be the saved file. (It uses the same pattern file name every time, so calling the script repeatedly wil not build up a large number of scaled pattern files.)