I might be inclined to disconnect the ign light connection at the alternator and use a test lamp one end connected to perhaps an unswitched live at the fuse box, the other end connected to the alternator ign light connection.... That way you will see if the temporary light, acts in a similar way to the original ign light in the variety of circumstances you describe, and might save you opening up your alternator to perform the diode checks...?
As ever just thinking in type and well past my sell by date on MGB thinking and much else..
That's a plan.
Unlike the old Morgan dash, the MGB is pretty enclosed and there's a fair bit of dismantling to do so that technique would see if that's necessary or not.
