I'm assuming you're talking about VR-4 calipers and rotors.

The non-turbo 3000GT calipers will bolt up. They are identical to GSX calipers except they use a different brake line fitting (I’ve also heard the piston diameter is 1mm bigger), but I believe people have gotten the proper brake line from RRE. And the rotors are the same as GSX.

I have thought of the VR-4 caliper upgrade plenty of times and to the best of my knowledge you are correct that all you would need is the right brake lines and custom-mounting brackets.

The problem is the custom-mounting brackets. Machine shop time is not cheap and designing the bracket won’t be terribly easy (it shouldn’t be that hard either). You’re probably looking at a few hundred dollars for the brackets alone. Then you still have the cost of the rotors, calipers, and lines. Meanwhile the AEM big brake kit will run you under $400 for relocation brackets and bigger rotors (I don’t recall if it includes new SS brake lines). The Bear big brake kit will run you around $800.

If you have the money and time and want something truly unique then go for it (as far as I know the only Eclipse with VR-4 calipers is a guy that goes by Widebody on DSMTalk who was sponsored by Quwees).

I believe Widebody had 3000GT VR-4 steering knuckles and hubs modified to fit the Eclipse.