Barry,
I like this idea as well! I would check out this post for a workaround:
https://community.skuid.com/t/grouping-logic-on-lookup-filters
Emily

Thanks Emily.  The workaround should suffice in most (if not all) use cases.  That said, its still a workaround and results in cluttering the models on the page and causing an unnecessary performance hit, etc.  Definitely a solution in absence of having the All/Any/Custom but I think adding those would be preferred and consistent with the logic on other type of “conditional” features.