How can I stop Keynote from trying to convert movies when opening old presentations?
Gah, I figured out a decent answer myself just after posting a bounty.

Simply create a copy of the presentation. Then right-click on the copy, select "view package contents", and delete all the .mov files. Now the copy will open with just a warning that some files can't be found.

(If someone can come up with a way that actually preserves the QC patches, e.g. by converting them into movies in a way that actually works, I'll award them the bounty.)