An adapter with a PCIe x16 or x8 adapter would obviously be more likely to work. MacOS should just see it as a discrete GPU. The smaller MXM-A cards require less heat dissipation than the MXM-B cards. But you would really need to think about cooling the card, as an adapter like that would lack any heat transfer elements or a fan.
You would just need to make sure the MXM card was compatible with macOS, the same we do for any other dGPU. Plus that it could drive the system at a suitable level.
I am not sure if your HD 7650 A card is compatible with macOS. I suppose spoofing the device-id would work if it isn't natively supported.
Here are two screenshots showing the cards used in real iMac upgrades (from AMD HD 4000. 5000 and 6000 series cards).
Nvidia MXM cards used in old iMacs (2009, 2010 and 2011)
AMD MXM cards used in old iMacs (2009, 2010 and 2011)
The Newer AMD cards being the better option, but also the ones hardest to source.
The PCIe x1 Riser cards are more akin to usage in Mining Rigs, as the adapters don't have any display connectors. So not really viable for a Hackintosh system. But definitely a more reasonable price.
If someone were thinking of getting one of the x16 riser cards they would be better served looking at an RX 6600 XT, which are currently available for less than the price of just the riser card.
Nice find, but at that price totally unfeasible.