I don't think any other calendar software lets you add events without a date -- where would it display them? Since BandHelper has a list view, that's possible -- it could display them at the top of the list -- but I'm afraid it would create confusion to have these floating there.
I'd encourage you to try it the way it's designed, even though that's different from how you're currently thinking of it. You would look at the Actions list instead of the Events list to see the leads you're working on, or use Settings > Appearance > Main Menu Shortcuts to allow them to appear on your main menu. Alternatively, you could create a special category for contacts like In Progress and filter the Contacts list by that category. But I think actions are better because they're dated, can send automatic reminders, and don't interfere with the primary categorization of a contact.
One other option if you really want to see these in the Events list is to pick a future date to park these on, like 1/1/2025, and then set the real date when it's available. You would not request responses or set the event as confirmed while it's in this state, and you might need to inform your bandmates to ignore events on this date. But if you use the Actions instead, you won't have to worry about people confusing this for a real date.