While I haven't tried this myself, partially due to the fact I can't get a Thinstalled messenger working in the first place, wouldn't it work if you took the snapshot of before WLM was installed, and then install WLM AND Plus!, and then packaged it all together? Because then theoretically you have all the correct files in place... It's probably a bit more complicated than that but hey sometimes the simplest solution might be the best one.
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