Do it in an XFBR, there you have everything you need in args.SubstVarSourceInfo.
Thank you Jack, that is super useful! I also managed to find what I wanted based on that object you provided in the api details. Pasting a snippet below for anyone reading.
Unfortunately it didn't contain what I was searching for now. I was hoping a dynamic calc could somehow pull cube view substitution variables from the cube view it is calculated in, but seems that it is not the case.
Dim instance As New SubstVarSourceInfoFactory
Dim dbConnFW As DbConnInfo = BRApi.Database.CreateFrameworkDbConnInfo(si)
Dim dbConnAppOrFW As DbConnInfo = BRApi.Database.CreateApplicationDbConnInfo(si)
Dim isSystemLevel As Boolean = False
Dim value As SubstVarSourceInfo
value = instance.GetOrCreateSubstVarSourceInfo(dbConnFW, dbConnAppOrFW, isSystemLevel)