Luckily for me, this appears to not be the case.You'll definitely need something to fake the RPM sensor. If the BIOS fan control software is trying to get the fans running at 10,000RPM and the sensor comes back slower it will raise fan fail alarms. If it gets multiple fan fail alarms it will shut down.
The DL180 G6 appears to accept any fan RPM (above a certain lower threshold) as "within normal range" regardless of what duty cycle it is currently requesting from the fans via PWM.
Right now, I have replaced the 8 sunon fans with 4 Delta 60mm 6800rpm fans. While on paper they are much quieter than the stock fans, they still are very loud (this 2U case must act as a reverberation chamber!) Temporarily I have swapped the 12v and 5v power lines cutting their speed to about 2500rpm, which is audibly tolerable for basement use (in other words, I can no longer hear it upstairs), and the server is not complaining about fan failure.
To make up for this lower fan speed I have zip-tied two smaller fans directly to the heatsinks like in a traditional client HSF. it's still a little close for comfort, since when loading all the cores up using prime95, core temps reached 61C in a 78F basement, without installed hard drives. Once I add my drives, and the basement temp goes up on hotter days this may be a problem, so I have ordered a fan rheostat to be able to increase the fan speed a little without going full on 12v and making it audible upstairs again.
When I reach a final setup that works, I will post pictures!