This little bit of Script-Fu converts all images of a specified extension in a single folder to greyscale .gbr files. script-fu popup prompts you for the folder containing the images, and which extension to filter on.
Brushes are created in the same folder as the images, using the original filenames (but with .gbr as the extension instead of frex. .jpg).
Tested in Windows XP with GIMP 2.6 - not tested in other environments or versions, but as this script is very bare bones, it should work on any GIMP version from at least 2.2 up (not sure about earlier).