Ah you mean going the Pi Port -> M.2 hat -> M.2 to PCIe riser route? I suppose that should work. Any bandwidth issues?

No, I literally made my own hand-soldered Pi Port -> PCIe adapter. That's to plug a 2x25Gbps programmable NIC, so no bandwidth needed because the NIC does all the work. All I needed was something to power up the nic :D

As for bandwidth, well, it's one lane of PCIe gen 2. This won't win any races but can be useful to access exotic hardware not available in usb or if you don't care about bandwidth. (e.g. HBA with many drives for mass storage without speed requirement).

How much power is the Pi port capable of delivering, or are you sending additional power to the PCIe adapter from somewhere else?

What SmartNIC are you using? Most SmartNICs that I'm aware of suck a decent amount of power, many more require significant external airflow. Are you using the Mikrotik active cooled one? https://mikrotik.com/product/ccr2004_1g_2xs_pcie

The Pi port can deliver 5 or 10W max at 5V IIRC. So I'm not using it :D

The 12V comes from an external power supply through a barrel jack, from which I also derive the 3.3V rail. The Pi provides no power whatsoever. I should publish the design files somewhere.

As for the NIC, it's a Netronome Agilio-CX which is fully programmable using eBPF and such.