Well Fish n fool, many of us do use the fiberglass resin as a molding material.
I have several and others make them too.
The resin does come off the lure. Might want to read up on several discussions previously.
http://tackleunderground.com/board/viewtopic.php?t=521&highlight=resin
http://tackleunderground.com/board/viewtopic.php?t=1015
Woodsac also had given more info about building the resin molds in a few other posts too.
Dont need to add epoxy or other sealer, lasts for a good long time. Just make sure that the master is nice and smooth so you dont get other dull baits.
Now if your looking to cast from a original master, You need to use Red's silicon mold tutorial. cast a few lures, then use them in the poly resin.
From what I have seen of plaster, It will set up in water, adhears to other materials. etc, But it is brittle.
I was just looking at a way to keep the detail that you get from resin molds, but make it cheeper if possible.