Matt said: "Because ArtRage files can be quite large and saving can be disruptive there isn't an auto save"

Here is my humble suggestion: the incremental save.

1. each time the user changes his work (as a stroke brush), ArtRage save this change in a small file (ie a partial file). This operation should not disrupts the user's work.
2. when the user save his work, ArtRage load all partial files to create the final file and then delete all the partial files.

It is maybe a solution...