Multiple subnets/single interface

I'm using m0n0wall v1.32 on a net5501 in a fairly normal configuration 
-- talking to a cable router with /29 static (5 usable IPs) on the WAN 
side, and a private network using 10.x.x.x/16 on the LAN.  All works as 
expected.

But every so often I get a new network widget with a web interface for 
configuration.  Invariably, the device has a default address of 
192.168.something.

It would be nice to be able to plug these devices into the same wire as 
the main network for initial configuration -- I would use that access to 
move them into the primary address space for production.

I thought I could use secondary IPs on the LAN to accomplish this.  The 
primary address on the LAN interface is 10.x.1.1/16 so I as an 
experiment I added a secondary of 192.168.1.254/24.  That would, I 
thought, allow devices on the two IP nets to coexist on the same wire.

However, this doesn't create a route between the two IP spaces.  Pings 
from the 10.x side to the 192.168.1 side get sent out the WAN interface 
rather than on the LAN.  How can I add routes so that the two IP nets on 
the same physical wire can talk to each other?

I know that this would be inefficient for full time use, but for this 
limited purpose of initial configuration that shouldn't be an issue and 
this would be a neat hack to ease configuring with new widgets.

Thanks,

John