This isn't a gimp plugin but I found this project from 2002Semi Causal Nonparametric Markov Random Field Texture Synthesis on a Gaussian Pyramid with K Nearest Search
The site has a little C++ app that can be downloaded and built on windows, linux or mac and then run from the command line, the results are quite good.
I started with this teeny image of rust (64x64)
and it created me a nice 512x512 texture like this
Using a single blob
It can generate something like this
or with slightly different parameters
If you like command line tools, you might like this. The cool thing about using command line tools is that you can set up a directory of samples, run the tool overnight and wake up a 100 new textures.