It's working great so far!
If I want to include tables, can that be done as part of the extension? Or would it need to be a module or rule-set or something?
My next steps:
1) Data entry of all the tables. I'm trying to contact the author (I backed the Kickstarter - Critical Codex), but I hope he'll consider making them available via Forge, or at least to his other backers.
2) Attribute/Tool checks. For example, Lockpicking. This isn't a 5E skill, so it would be a DEX check (using Thieves' Tools). I assume I can override the attribute check, but I'm not sure yet how I would do so just for lockpicking. Perhaps adding an Action to the character? Or maybe the extension would only apply the custom code if some condition was met (e.g. Alt-roll, or whatever). That would probably be fairly easy to implement, but not very intuitive.
3) Saving Throws.
4) Combat (Attacks and Spells). I assume I could do this based on Trenloe's Nat20 extension. Ideally, though, it would use different tables for different weapon/damage types (at least for bludgeoning/piercing/slashing).