Hmm... Maybe I'm just completely missing the point but,
"and dynamically change the properties of other such, properties according to layer parallax,"
You'd just basically pretend that "VarParallax" IS "Layer.ParallaxX". Though I can tell you know what you're talking about so maybe I'm really missing the point lol.
If it's the case of that you have like 100 layers with individual parallaxes or something, then yeah this method would be pretty crap since it would take a long time to create individual variables for each Layers' Parallax. :P