(me ducks for cover) you can use filter -> other -> offset
to get the efffect of getting the edges of the tile in the middle while having the middle at the edges to be able to paint away the edgetiling (did that make sence ?)
You mean this effect, of course:
In gimp, that is layers->transform->offset. And it includes a 'offset by x/2, y/2' button which is exactly what is needed for tiling.
Reminds me.. time to create a little plugin that does that without the dialog popup so I can just shortcut it..
The effect only applies to the current layer, so you can float a tile out of a tileset to work on it with the help of offset.
