For the longest time I have wished that Complex Equations could access fields in connected tables. But they never could - and I had to make a text/formula field just to bring in a connected field into the table for Complex Equations to be able to use.
But … today … seemingly unannounced I can connected fields available when adding fields in Complex Equations. I got excited, but I cant seem to get them to work. I’m not sure if its a new feature or an error.
I just ran into this same thing today. I was very excited but then the function equation was not validating.
@tim.young and @moe is this something that is being released or it is a bug and we should not be seeing the connected fields?
It is unfortunately not intended 
BUT stay tuned to our updates page as we are experimenting with some new functionality for connected fields.
Crazy fast response. Thanks for the info.