Spell Mastery Feat?
 

When I select the Spell Mastery Feat it complained that I needed to specify a Domain. It took me a little while to figure out that putting any free-form text in the "Details" customization made this text go away.

After that there seems to be no spell selection for the feat to make what spells can be prepared without a spellbook. I'm wondering if this is a bug or if I should just make a "Spell Mastery Spellbook" to represent the spells I can prepare as a workaround?

Ah yes, "Domain" text is the user text field. We should really change that message, I'll make note of it. As for selection, there is none. In HLC it was set up that you specify your spells in that field, and nothing changed there when we converted it all to HLO. Adding a Spell Matery spellbook is the best workaround currently, I'll log this as something for us to potentially look into, it might be a straightforward thing to add this to the feat itself but I'm not sure.