I've seen software to link a MBP's keyboard to other devices, but is this possible for MBP's trackpad? I want to be able to use my MBP trackpad with my iPad. Even if I have to develop the software myself, where would I start?
Yes, it is possible to relay the MBP's trackpad inputs to other devices in general.
Using the MBP trackpad with any iPad and in general is unfortunately not possible.
If you want to develop the software yourself, you would be looking at creating two applications:
First a small application on the Mac that captures the trackpad input similar to how VMware and Parallels Desktop captures trackpad input. It needs to forward it to the iPad using for example a network protocol.
The second application is an iPad app that speaks the same protocol and thus is able to take in the input from the trackpad. You would then react to that input in your application like you react to touches from the user.
You can use this open-source project to inject the trackpad input into your app:
However take note that this will only work with your own app. You cannot use the trackpad input to control other third party apps (unless you have convinced them to use your protocol), or to control Apple's default apps.
If you're willing to jailbreak your iPad, you can control other third party apps and Apple's default apps by injecting your trackpad inputs into the system.
You could even use this open-source project instead of building your own iPad app to inject the inputs:
It features a simple network protocol that you can send trackpad inputs to and have it injected as touch events on the iPad.