OK, thanks for all the suggestions! I didn't know what ferrotyping was, so I had to look it up, and learned something new on the way, although I don't have room for that kind of extra equipment in my current home.
I have now sacrificed one of my prints for "science sake" ;-)... I have attempted to coat it with my oil painting varnish. Although in terms of gloss, it kind of OK, and one is able to achieve both deeper black and a gloss a bit similar to "glossy" fibre based paper, the big problem is to get an even coat.
On oil paintings, with their brush strokes, any uneveness in the varnish layer, is invisible. However, on a smoothly coated commercial printing paper, any uneveness in the varnish, is immediately and disturbingly obvious.
It probably requires diluting the varnish with turpentine, and using a very soft brush, to get an acceptably even coating.
Anyway, nice to give it a try once, but I will reprint on glossy paper when I have a chance.
Marco