The back fans may still run as there is connection to the motherboard, but without firmware support there is no control over speed. It may or may not provide adequate cooling as the BMC does not recognize them. Page 5 provides more information on this from a system perspective: https://downloads.dell.com/manuals/all-products/esuprt_ser_stor_net/esuprt_poweredge/poweredge-r900_...
Thanks for your reply.
My memory risers are getting very hot these days so I think the back fans might help to cool it down. I have seen that the 6 pin connectors are still there.
In my understanding, the fans are driven by DC motors that do not know anything - they run as long as 12V voltage is applied on them. So this really boils down to, if the BMC is disregarding the ports for the rear fans, is power still supplied over there i.e. is there still 12V across the 6 pin connector? If there is, the fans will run at top speed I think.
If anyone has tried something like this before, I would be happy to hear the outcome.