I was struggling with GtkBindingSet stuff (rather poorly documented and without usage examples) when I found that GTK+2 has an easier way to do that through .gtkrc-2.0:
binding "custom-text-entry"
{
bind "<meta>v" { "paste-clipboard" () }
bind "<meta>c" { "copy-clipboard" () }
bind "<meta>x" { "cut-clipboard" () }
}
class "GtkEntry" binding "custom-text-entry"