Ever since Portable Gaim beta came out, I've been waiting for Portable Gimp 2.3.13 (or whatever version). The betas of GIMP add heaps of needed features, and are actually quite stable. I don't know what I'm doing, but I wouldn't guess that creating Portable Gimp 2.3.13 would be too hard, considering that you've already created Portable Gimp. A couple version ago, I tried compressing an install of Portable Gimp 2.3.x with UPX and copying it over, but it didn't work.
Gimp 2.3.x release stream is considered unstable. There was a discussion here a while back about doing a test release, but the concensus was to wait for 2.4 official.
On the other hand, I have been using 2.3.11 since the end of September and have only found a few plugins that do not work. The base functionallity (what most people use for production work) is extremely stable. What I like the best are two things. 1) The menu structure has been reorganized to be more logical. 2) EXIF data is preserved.
The down side is that it loads slower
I have been able to create a portable version of 2.3.11 by installing 2.3.11 and copying the resulting install structure over top of John's Portable Gimp release. I think I has to do the same with GTK but I don't remember for sure. I tried to compress the exe's but couldn't get that to work.
I am considering updating to 2.3.12 (http://sourceforge.net/project/showfiles.php?group_id=121075), but I expect a new release within the next couple of weeks, so I think I'll just wait and do that one.
Hope this helps,
Clair