Custom Groups: Multi-select "used for" subcategories

When defining a custom group, options are given for defining it's use based on categories and subcategories. For the subcategories, it would be helpful to be able to multi-select options. Currently you can select a single option or the "Any" option.

Example:
Create a new custom group for contributions. Subcategory lists the contrib types available. I would like to be able to select 3 of the types.