Thanks for the reply, Tony!
My approach was naive: using the Tuner web editor, I entered 🤔as the key combo for a chord. I'm using macOS Mojave. I do see the emoji in the Tuner when I refresh the page.
I'm not familiar with the nuts and bolts of HID codes or emoji implementation, but if I understand it, I need to figure out a sequence of HID codes to add the relevant Unicode character on my OS. Does that sound right? Is that feasible?