Hi,
This shouldn't be an issue, and secondly you shouldn't get any out of bounds error. Anyway, its easy to add what you need. In tk2dSpriteAnimationEditor, look for AutoFill. Thats the function which does the autofill (from the current frame). All you'll need to do is get an upper limit and pass that in, and right after FindFrameIndex, break out if it is >= that number.
Let me know if that doesn't make sense and I'll try and paste a patch - only problem here is I've just been making a ton of changes in that file, and whatever line numbers I give you is likely to be irrelevant depending on the version you're on.