So I'm kind in the same situation but for me it's not working.
I separated mplane from dplane according to the (poorly documented) sk138672.
Right now the management plane is isolated which is good. BUT as this is done is software I have some strange issues:
Packets originating from the management interface traverse the management plane and lands on dplane to be processed by the firewall. dplane recognise the source IP and it's marking it as spoofed. if MDPS is to fully isolate the network. This breaks almost everything like DNS, AD for Gaia LDAP AD binding, TACACS. SMS still works because due to an "error" is in the same network 🙂 but otherwise it will fail.
THe inbound traffic originating from inside the network (from one of many internal interfaces) arrives in DP where is processed but due to "mdps_tun" the traffic is sent over to mplane. Of course, as MDPS has a default route, traffic is sent over the default route, which lands on dplane and flow is broken due to symmetry issues.
So basically from the internal network I cannot access the management interface).
I know it's software but still. MDPS should be a real isolation.
I'm thinking on switching to a dedicated VSX just for managemnet but.. as everything is in place right now, removing mdps will be a mess.