a more basic issue is the knock-on effect this would have on non-hacked sony readers i.e. those NOT running PRS+
there would I think have to be 2 version of the plug-in, one for normal SONy's - placing the images where they are urrently located to be used a cyclic screensavers,; and a special PRS+ version as per your draft spec above.
I've no idea of the ratio of normal sony users to PRS+ users, I somehow doubt there's be sufficient enthusiasm for maintaining this as it would need ongoing co-ordination between at least 3 developers - 1. jackie for plug=in, 2. Kovid for calibre + 3. whoever for PRS+
It would be simpler to make the whole thing a PRS+ stand-alone feature: extract cover art from books on reader, use as screen saver i.e. do exactly what Kobo firmware already does. OR dare I say buy a Kobo if this is a must have feature!