I ran ipmitool and I didn't see any fan speed from the built-in fan hub of the Fractal case.
I decided to try replacing one of the default 3-pin case fans with a 4-pin PWM Noctua fan
If the fan is at the rear of the case, the 30cm cable from Noctua doesn't quite reach the fan connectors on the front of the motherboard.
The fan hub has some 4-pin connectors and 3-pin connectors. It appears that it takes the speed sensor from the first 4-pin connector and passes that back to the motherboard. Connecting the fan to the first 4-pin connector, I was able to see the speed with ipmitool. I tried moving it to each of the 4-pin connectors and they all report 0, it only reports the speed if it is on the first connector.
Using the fan hub in this way also solves the 30cm problem because the fan hub is at the back of the case too.
At idle, I don't believe the system is significantly quieter using the Noctua fan. ipmitool reports that the fan is running at its slowest speed, 300 RPM. I suspect that any noise I can hear is coming from other parts of the system and not the Noctua fan.
Overall, it is convenient being able to see the fan speed like this but maybe I could see the fan speed by using a more advanced fan hub that can monitor the 3-pin case fans.