I suppose I'd probably start with identifying the trackpad in question, figuring out what interface it's on.
On the Linux side, probably sudo lshw will give you the information somewhere. On my ThinkBook, looks like the trackpad is exposed as a PS/2 device, but I have no idea what interfaces Macs use for their trackpad. Might pretend to be a USB device or God knows what.
It does sound like people have made 2012 MBP trackpads work -- here's someone complaining about having a problem with his, but it's clearly recognized and functional.
Considering that you apparently already are needing to use an external screen, though, a suggestion for an alternate approach, especially if you can't give it working.
I've been increasingly irritated with how "closed" the laptop form factor is and with its technical limitations (can't put out a lot of heat, can't expand battery capacity) and expandability. One thing I've been thinking about doing is maybe using a small-form-factor PC and just carrying a portable monitor and input devices. Less portable, but has a lot of benefits in terms of hardware flexibility. I can choose the monitor size I want, expand the battery, expand memory, can choose the keyboard I want, can choose the mouse I want.
The keyboard and trackpad are one issue to address there. I was thinking about was maybe just using a keyboard and trackpad. I don't know of a great solution to attach a keyboard to a trackpad. In my case, I have a better option -- just use a trackball. That works even if you're lying on a bed or something -- I'd like the thing to remain usable in that situation -- and a trackpad is, in my testing, the device which gives me the least-accurate-input of any of a mouse, trackball, or trackpad. I don't need a to use a trackpad if I'm not constrained to the laptop form factor.
But if what you like is the "trackpad+keyboard" combination -- which does have some benefits, like slightly-faster switching between the mouse and keyboard, there are two other options:
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Get a keyboard with an integrated trackpad. The options here are more-limited than getting a dedicated keyboard or dedicated trackpad, but they do exist. Some have the trackpad off to the right, but some have them in the center.
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Possibly just figure out a way to affix the trackpad and keyboard to a surface. That does have the benefit of letting you use whatever trackpad and keyboard you want. You might need to put something beneath whichever is thinner if you want a flush surface between the two.
That means another piece of hardware, but it means that you can shove the laptop off somewhere else rather than having it sit under your wrists, and I assume that if you have an external monitor already, you've already accepted some extra set-up time if you're moving the system around.