I'm adding expression drivers to my modifiers structure. One of the variables I constantly use in different modifiers is the number of frames. Is there any way to attach the driver to a custom property that will be located in my modifiers stack so I see it while working with other parameters in my modifiers? The modifiers that I am using don't have a custom properties panel. But perhaps there is a trick?...
Why not just use a custom property of an object? You can see it and modify it in Numbers Panel while working on modifiers:
(after creating the property simply right-click on the field and choose "copy as a new driver", then paste that driver in your modifiers)
Also remember you can setup a layout where you can see the contents of multiple instances of the Properties Editor: