Pretty much every SoC has cores equivalent to the management engine. Currently they have a handful of ARM cores on the southbridge that fulfill the same purpose. I imagine that won't change. "BridgeOS" is the term to search for if you want to learn more.
The thought experiment is probably moot anyway though, as Apple probably won't allow any kernels that haven't been signed by them to be booted like on their iOS devices.
I ended up doing a bunch of research into and asking around about the T2 chip (which seems to be the closest thing to an IME Apple advertises) today and got a variety of responses.
The general picture I've gotten is that the T2 is probably significantly less capable of surveillance than, say, the IME. This talk [1] for example suggests (but doesn't rule out explicitly as far as I can tell) that the T2 is not connected to the PCIe interfaces for network cards, which significantly reduces the extent to which the T2 could autonomously phone home what it could learn through its direct storage access and connection to the CPU.
And yikes! No unsigned kernels would be pretty bad. I certainly wouldn't be buying if that's the case :(
The thought experiment is probably moot anyway though, as Apple probably won't allow any kernels that haven't been signed by them to be booted like on their iOS devices.