Further to this, I'm wondering if anyone is actually using the micropayments code successfully (to switch between a regular PayPal account for higher-value payments, and a micropayments-enabled PayPal account for low-value payments, to minimise fees)?

When I searched the forum logs, I can't find any mention since 2013. It seems like such a useful feature that all PayPal users should be using it to save money on fees. Is everyone using it successfully, hence no posts on the subject, or does nobody use it at all?

I'd be grateful to know if anyone has managed to get this working on the latest version of Zen Cart, just so I know it is possible!