Edit: Resolved to do this

https://drafts.csswg.org/css-ui-4/#widget-accent

As discussed in today's telcon, there should be a keyword (akin currentColor) or other way to refer to the value of accent-color so that authors can use this in their designs as well.
The advantage of this over a custom property, is decoupling: that libraries can hook into existing code using accent-color (especially given proposed guidance to set it on the root), without the code needing to be aware of the existence of the library.