Well, it's arrived!
Here it is plugged into my A500, with an ACA500plus on the pass-through with an ACA1221 on that. Plugged into the Zorro II slot is a GBAPII++ graphics card, the one developed by Matze from A1k.org.
For what concerns the Z-500, that worked out of the box. The GBAPII++ had / has a few issues, which I am still tracking down. Any video setting higher than 800x600x8bit gives a weird ghosting and other artefacts. I'm not yet sure where the problem lies, but I'm pretty confident it isn't the zorro adapter.
I'll do a full review writeup this evening or next.