Maybe this will surprise you, but nothing is required to get existing components listed in the property editor of a component property in your own component. Just declare the property as the desired type, and the VCL framework will do the rest.
For example, consider this very simple component:
type
TButtonSelector = class(TComponent)
private
FButton: TButton;
published
property Button: TButton read FButton write FButton;
end;
After installing this component in the IDE, when you select the Button property in the Object Inspector, all existing buttons on the current Form are listed.
This is all build in in DesignEditors.TComponentProperty
, which means that the only requirement is to let the object you want to select descent from TComponent
.