Toolbar Customization offers classes that allow some toolbar customization in REALbasic 5.0.

Warning: Its not a complete solution! Thanks to REALbasics design of toolbar item classes, its extremely difficult to allow StandardToolbarItems in toolbars and would require a near complete rewrite of everything Ive already written. Besides that fact, its impossible to detect a click on a standard toolbar item in the toolbar which would make it impossible to customize standard toolbar items in the same way you would a custom toolbar item. Thanks to this, Ive completely stopped working on that idea. Any standard toolbar item code/classes you see in the project now was an attempt at a work around, but it has its drawbacks and I simply dont want to continue working on it if its only half useful.

So with that in mind, I present to you the project. There isnt any documentation, but the general idea is, drop the classes in the "Toolbar Customization" folder into your project, make your toolbar-using window a subclass of the ToolbarUsingWindow class, and then create a customization sheet as in the project. Its not difficult at all to implement. All you as a developer need to look at are the two windows and their containing controls: Main and CustomToolbarSheet. Thats it.