[Phanx]

Originally Posted by Duugu New idea: I'll query all flyoutIDs up to ... whatever ... 1k and query and store all slots for flyoutIDs that exist on addon load.

That seems like a lot of unnecessary scanning. Just scan the player's spellbook for the flyouts that are actually available to them.

local _, _, offset, numSpells = GetSpellTabInfo(2)
for i = offset, numSpells do
    local skillType, skillID = GetSpellBookItemInfo("spell", i)
    if skillType == "FLYOUT" then
        local _, _, numSlots, isKnown = GetFlyoutInfo(skillID)
        for j = 1, numSlots do
            local spellID, overrideSpellID, isKnown, spellName, slotSpecID = GetFlyoutSlotInfo(skillID, j)
        end
    end
end

(I'm pretty sure nobody has any flyouts on the General tab, but if they do, just start the i loop from 1 instead of offset.)