Route the management vlan externally

[NomAnor]

I've got a 5406zl switch which should route multiple vlans (10, 11, 12). The access to the configuration interfaces (http, SSH) should be restricted by a firewall. So the management vlan (20) must be routed by the external firewall. The "Advanced Traffic Management Guide" states "Routing between the Management VLAN and other VLANs is not allowed."

I have to set an IP address for the management vlan in order to access the configuration interfaces. This automatically creates a "connected route".

If i ping the switch from VLAN10 the switch routes the packet to the management vlan and then filters it (ping returns "Packet filtered"). The management vlan is isolated as it should but I can't route it externally through our firewall.

Did I miss a configuration option or is it not possible to route the management vlan externally?