I just keep all my textures in a directory, then after an update drag and drop them manually into the jar using Winzip. Takes about a half hour.
Which is why I don't bother with the pre-releases.
"The way I see it, every life is a pile of good things and bad things. The good things don't always soften the bad things. But vice versa, the bad things don't necessarily spoil the good things or make them unimportant."