ROM is just a generic term for permanent storage, which usually makes it easy to find. It's just like finding the TSOP on an Xbox, it's obvious after a bit of detective work.

It's generally a large chip, although Apple likes to confuse by putting a ton of large chips on the board... It'll be a unique manufacturer most of the time. The package itself has markings on it which indicate exactly what it's intended purpose was, and usually a company logo. With those things, (and a macrophotography lens wink ) you can figure it out.

But even so... It'd require hacking and flashing of that chip, because it's a longshot the firmware from the newer macs would even boot on the Beige. It's not worth the time or the effort. You'd end up with a big Beige brick.