Have you managed to do this? I am looking for exactly the same thing
Actually it should be possible; when using a tile layer which only has zoom levels 17 and 18, I keep seeing it for a very little while after map enters zoom level 19.
During that fraction of second, the tile layers are still visible, and zoomed (they are increased in size) ... but then they dissapear!
So I guess there should be a way to keep them visible. Any hints?