Gorm the extension uses the Flarum settings key-value store which is indeed limited in length. Changing the type of the value column on settings shouldn't break anything, but I'm unsure whether this has any performance impact on the other Flarum features.
The extension wasn't really designed for the purpose of loading externally generated domain lists, I believe the Anti Spam extension has that feature built-in.
Adding validation rules using an extender is very easy, in fact this extension barely contains any code. The most performant solution to load a custom blacklist file would be to write your own validator that reads from a file instead of the database, and add it from the root extend.php.
If I ever work on this extension I'll check if it's reasonable to add an external data source option, but this is definitely not a priority.
