There was a thread on photo.net not long ago in which someone described
embedding a silver image within a layer of anodize on aluminum.  He used
printing plates, anodized but did not seal, then sensitized with a
silver nitrate solution and converted the silver to halide in the pores
of the anodize; developed, fixed, and then boiled the plate to seal the
anodize pores, effectively locking the image inside a layer of synthetic
sapphire.

Should solve the problem with sliding off, though you might find the
sulfuric acid anodizing bath is more than you want to deal with...

If you don't want to get into anodizing, you might try wet sanding the
lacquer coat and then applying a second subbing layer of straight
gelatin (Knox from the grocery will work for experiments, but you'll
probably want to buy photographic gelatin if you do much of this) before
applying the emulsion.  I'd guess the gelatin just can't get a grip on
the lacquer as you'd doing things now.  Alternately, and simpler, you
could try cleaning the lacquer surface with chlorine bleach (with a
distilled water rinse and air blast dry) before applying the gelatine;
that works well for people applying carbon prints to glass.