if you bend a disc on the rocks, your stuck with a bound up driveline
the stock drum handles some abuse

I don't see why you'd bother with a disc setup when the standard drum is 100x easier, far cheaper, much less susceptible to damage, wont be picked on in a pit inspection, has off the shelf parts available and is already very effective. Who's to say a homebrew disc setup with a Chinese motorbike caliper would be more effective anyway? I doubt it would be. You're not going to get any greater brake diameter, the pad surface area would be significantly less, I doubt you'd get appreciably greater force applied and a home made disc is going to be less than optimal.

I think the only advantage of the disc is lower rotating weight, you'd never notice the difference though.

Also, I wouldn't use a hydraulic setup as my handbrake. If I was forced to, I'd pick line locks over a transfer mounted hydraulic disc setup anyway. It'd be likely be cheaper, certainly easier, guaranteed to be effective and it wont give you the issues that open diffs and a transfer handbrake do.

The standard transfer drum brake works awesome. If yours doesn't then it probably needs adjusting or the pads are worn out. If you can pull more than 6 notches on the handbrake then it requires adjustment.

You can use an SJ40 drum by creating a hybrid 1.3l/1l uni joint and using the 1l tailshaft flange. 1.3l NT is a little easier, just swap the tailshaft flange as jono says.

I remember someone on here saying you can fit the LJ transfer brake to a Sierra transfer. Might be a good option seeing as an LJ transfer is worth about $5. You'd need to run hybrid unis like a 1l though (LJ is the same bolt pattern as 1l).