They work in R10.1.1 10.1.2 broke them. 10.1.3 fixed them. You need to add the allow_5410_upgrade in the nouser source codes and provide an external TFTP server if you have 5410. Fill in the IP in system for the TFTP. 5420 should upgrade on there own but they may have issues displayying the time. I have a 5410 connnected to a R11 here in my lab and haven't see any issues yet. I also have a 5420 working without issue at 11. But yes... it isn't supported, but as always, doesn't mean it doesn't work.
Dermis and feline can be divorced by manifold methods.*
*(Disclaimer for all advise given)--'Version Dependent'