Hey! Checking in from Finland with the same request/need for dynamic pricing. From previous posts i have seen that at least Denmark has a similar scheme and Germany will have it going into 2025. But there are probably other countries as well.
I believe the addition of the modifier you suggested would work well, provided it supports standard operations like addition, subtraction, and multiplication, just like other parameters in the formula. It should also allow modifications to the table for different time slots, as per your suggestion. This flexibility is essential since some countries, like Finland, have day/night pricing, while others, such as Denmark, use multiple rates throughout the day.
Hopefully you’ll have time to add this feature, I have been hoping and waiting for it since launch 
Edit:
Post about Germany: Variable grid charges in Germany from 2025
Post about Denmark: Dynamic tarifs in DESS (Includes reply about Poland having it also)