By the wonders of modern technology my iPad is now "on my LAN" when I am out and about. I simply have one of my Data-SIMs set to relay to the FireBrick at the house which routes on to the LAN. Works a treat.
Although the iPad is not IPv6 when on the mobile network (watch this space on that one) the L2TP relay to my house is IPv6 - which is a slightly backwards world.
P.S. I cannot give it the same IP on WiFi and Data-SIM. Well, technically I can, but as the Data-SIM is a /32 route it means all traffic to it would go via mobile even when using the WiFi, which would not be good.