Usually you have to right-click over empty space on toolbar, but sometime's that's a bit tricky, so my suggestion is to always show "Customize...", "Float" menu items, even when right-clicking over button, in non-Customize mode.
Much like dragging/dropping on bread-crumb bar - somerthing that didn't happen in Opus 11, and is something that always annoyed me, but is now present.
Hope you'll consider it, unless there are any reasons not too ?
I doubt this will happen, and I would not welcome this as it would break "3-button" feature where you can stack 3 different operations over 1 button (left, right & middle click trigger their own commands/scripts).
EDIT : If you want to enter customize mode, you already can do so by selecting that entry from the settings menu (or copy that button to wherever you want on a toolbar) or you can alt-(left-)click anything in toolbars (might need some setting to be switched on).
Preferences / Toolbars / Options / Alt + click to edit toolbar buttons
Or make up a button with Prefs CUSTOMIZE as the command.