Need a way to do a multiple select field on the line level of a transaction

I have a specific need to record values from a multiple select list on a per line basis. Since you can't create a multiple select custom column field or custom item option, so here I am for help.

Use case is we provide product to outside sales at a heavily discounted rate so that they can demo them to prospective customers. This is done today on a Sales Order, and ideally would stay that way. We're now asking for more transparency, so the idea was that each line could list the end customers that are receiving the samples. Would have been easy enough to do, but for some reason I'd never noticed that multiple select lists aren't available on the line level.