Assign IP to network device

I have a network device (LED Sign) that is shipped with an IP of 192.168.X
I need to write a utility for my customers to assign a new IP to it. The sign has a web based setup screen.
My problem is that my customers may have a 10.1.X network.
They setup the sign by connecting directly to it via a crossover or through a hub.
I know the MAC address(serial number) of the sign.
I set up a small 10.1.X network with a router and gateway and plugged my machine and the sign into a hub to simulate a customer.
I tried adding an entry in my machines' ARP cache as 10.1.1.100 and then pinged 10.1.1.100 but no joy.
I have been scouring the net and reading anything and everything I can find that even remotely is in the ball park, I've learned more about networks and IP routing than I ever thought I would but can't seem to get my head around this one.
Does anyone have any ideas on how I can get to a 192.1.X device from a 10.1.X device with a known MAC address?

Thanks for any Ideas

 

yes the sign has a broswer based as well as telnet and FTP methods for setting the IP