Never mind - I had not thought the 5V-to-3V3 conversion through. While 5V-to-3V3 conversion is straightforward, things get complicated when trying to use the same circuitry in a 3V3 system - it requires far too many jumpers. Adding to that are the other pins on some of the ConnectOne modules, SPI, RMII, etc. which complicate things even further.

So I'll just go back to a silkscreen warning that the ConnectOne shield is 3V3 only. It will work with the Amicus18 and other compatibles with 3V3 system voltage.

OTOH, the Tibbo ethernet shield is easy to use with either voltage, requiring only a single jumper to select the system voltage so I will add an LDO regulator to that PCB.

An interesting side note is that the Arduino Uno reference design shows optional 5V and 3V3 regulators so there are probably a lot of Arduino compatibles that are 3V3.