It wouldn't be supported by Dell, but wouldn't void the warranty. However, if you ever have issues with the MD1200, support may require you to pull out the H800 and let them test the setup with that.
Have inherited an NX3100 and two MD1200 with four 149 GB SSD and twenty 600 gig 15k SAS drives. I was not able to enable Cachecade as the PERC H800 in the NX3100 is only a 512 mb battery backed memory. My questions are: Can this PERC H800 be upgraded by installing the 1GB NV module and the necessary firmware ? Will Cachecade 2.0 that enables write cacheing be available to upgrade if I have to purchase a new PERC H800 NV 1GB that already has Cachecade (1.0) in firmware? Does the PERC H810 already have the Cachecade version 2?  If it does will it work in a Dell 510 server?  
NX3100 and MD1200s with SSD and SAS drives - enabling Cachecade