I'm interested to know if this is on the cards.

Seeing as UEFI is about on Macs, I should theoretically be able to do this by selecting a different OS.

This question is currently low priority. But that may change soon. It's also useful to know: Are Apple friendly or hostile to linux?

I frankly don't bother with "dual booting" anymore. I run my Macintosh the way it always runs. Then, I use VirtualBox® to host Linux (and Windows) as a virtual machine. All of today's modern microprocessors have specific hardware support for this mode of operation, so there really isn't a speed penalty.

While VirtualBox is certainly not the only available "virtual machine monitor," it is: "full-featured, absolutely free, runs on everything, and is backed by probably the largest software corporation in the world."

All of today's popular operating systems – Windows, Linux, Macintosh, etc. – are now engineered to "work just fine" in a virtual-machine environment, no matter which "monitor" you decide to use, because a substantial percentage of today's so-called "cloud" is built upon it. (And: the rest is built upon "containers.")

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I used REFind to dual boot Linux and OS X for a while. I don't anymore.

it is indeed possible to install Linux alongside macOS or replace macOS entirely by selecting a different operating system during the installation process. Macs with UEFI firmware generally support booting into different operating systems, including Linux distributions.
However, it's important to note that Apple's Macs have unique hardware components, and Linux compatibility can vary depending on the specific model. While many Linux distributions work well on Mac hardware, there can be occasional challenges due to proprietary hardware drivers or specific configurations. It's advisable to research and ensure that the Linux distribution you choose supports the hardware components of your particular Mac model.