Given how the code is structured in that area, this is a deceptively difficult problem. As MrDDT said, I believe the new Trigger Me Timbers solves this.
Printable View
Figured I would post a screenshot on how to do it in TMT; I did this for relentless Endurance, but you can change it to whatever you name you want, You can also do it with a trait vs an effect. I like using the effect so it can remove it once it's done and only have it come back on a long rest.
Attachment 55910
Version Update: 2.65
Fixed: Effects Builder DMGCS and SAVEO
This is a shot in the dark since it's not exactly what this extension is for, but would you consider coding a new effect that allows you to add/subtract from the proficiency bonus, such as "PROF: 1" or what-have-you? Currently, nothing lets you interact with it, and there are some fun things one could build into games if they could unlock that number, like skill drain features similar to older editions.
It is not. The ability to fully affect one's proficiency bonus is what I'd want. I should have said level drain rather than skill drain (you can already affect skills). I know it's a niche ask, but it would unlock a function that, strangely, FG doesn't already allow.
Version Update 4.0 - Refactor
Feature: BCE saves support (RA) which will remove the effect on a failed or successful save
Feature: DMGR will now stack with itself
Under the Hood: Moved signifgant code around to more fall in line with the FG paradigm and pipelines. Should be easier to maintain and less prone to errors
Under the Hood: Added a binary sorter for better performance when searching custom effects window with a lot of effects.
If you are an extension dev and are hooking into BCE, you are likely broken
Version Update: 4.1
Fixed: Saves for Pathfinder not working
Found a conflict with this extension and Kelrugem's Full overlay and automation
https://forge.fantasygrounds.com/shop/items/1/view
The conflict involved rolling attacks and damage.