Interesting idea! Strum Machine actually had something like this way back in the early days, before lists were introduced; there was just a big list of songs in alphabetical order, and you could “star” them to make a list of them. The “lists” functionality replaced this to offer more flexible organization.
I have made a note of this idea. It does bring to mind another idea I had, which is for bulk song selection (“checking off songs”) in the song index and the ability to quickly do things with all selected songs, like adding/removing them from a list. I do recognize that is not equivalent to what you’re suggesting but it may help in some way.