I finally figured it out, I think. Adding how I did it here in case anyone else has the same question, since the existing Ability System docs are useful but lacking a lot of specifics, at least for my use cases…
So…First, in your GameplayEffect, you have to specify the Modifier Magnitude to be Set By Caller instead of scalable float. When you do that, you’ll be able to edit either a Data Tag, or a Data Name. (in theory…Data Name was non-editable for me, but I assume you SHOULD be able to edit that…)
You can then reference that Data Tag in your BP when you apply the effect, like this:
Char is my Character, which has an AbilitySystemComponent of course.
However, note that I was originally using ApplyGameplayEffectToSelf instead of ApplyGameplaySpecToSelf…minor difference in some cases, but I couldn’t find a way to get a handle to the GameplayEffectSpec using the simpler method, so I had to use that Make Outgoing Spec function to get the spec handle, which is required for the Assign Tag Set By Caller function.
Anyway…hope that helps someone else trying to figure this ability system out…