I knew a guy back in the day (2000-2004) who plays Orks who did this. He bought the models and painted them a simple 3 color scheme for tourneys, but for regular play he'd always field the strangest most detailed and ridiculous proxies.

Fucking soda can with nailed on wheels and scrap brass fittings. Dude would root around the trash / ground at construction sites for electrical off cuts and things, then go home and make them into models to field, it was awesome. He's the only person I knew that used the sprues from his models for bitz.