I'd like to add accelerators for GtkComboBox::move-active or GtkRange::move-slider
Therefore I would need to specify extra parameters for the signal handler to gtk_widget_add_accelerator(). There docs right now don't tell that the mechanism works only for parameter-free signals (like GtkButton::clicked).

The actual problem is the underlying AccelClosure implementation in GtkWidget.
It wouldn need to copy the parameters in widget_new_accel_closure and in closure_accel_activate probably use g_signal_emit_valist instead g_signal_emit.

It probably needs to be done as separate function (bah, gtk_widget_add_accelerator_full). At the same time the gtk_widget_set_accel_path() function should be revised as right now this imposes even harder limmitation - it only works for the activate signal.