@cliffwoodjames Sure. Though before I do, you should note that since my post, there’s been a popular plugin released which seems to do exactly what we were looking for, and more… bad timing for me, maybe better for you.
Selectize Dropdown: [RELEASED] Selectize dropdown
And I now see this from 2016: [New Feature] Multiselect dropdown & tags
Check those out and see if you’re still interested in my workaround.