Well, there is an option I brainstormed last night. You could keep the limit of 16 "normally added" secret base items. Then maybe manually script up a way to place more, but those would only be on your game, that data wouldn't be transferred when mixing records.
So you'd have to get a script that gives the option to place more OWs dynamically, and then save them through use of flags or variables or something.
Though I don't know if 16 is a "display at once on screen" limit or like a "can't save any more cause there's no more space in memory" limit.
OR: You could add more sprites AS secret base items, but put two decorations in one sprite. So for one OW, you could put two decorations in it. You'd just have to make sure the palettes are similar. So two blue Pokemon could be on one sprite. That way, you could give the illusion of having more base items, when really you still have the 16 limit. That also would work around the 16 display limit, if in fact I was right in assuming that was what Jambo implied.