how about using a relay attached to your ignition switch to switch
the +24v to the ACC wire?
In other words, -12V, +ground to inverter, +24V from inverter to
unswitched headunit input, +24V from inverter to relay to swtiched
headunit input, and -12v, +ground from ignition switch to relay.
I'm assuming relays are all electrically isolated, but you may want
to confirm that (and in any event it would be much easier to isolate
one than a headunit).

 

Bill, there are two power inputs into the HU:

a constant 12V input (for memory and such)
a switched 12V input that comes from the accessory line

a simple way to wire this would be to have both wires run from a single converter and use a relay switched off the acc line.

Split the converter output:

1 side goes directly to the constant 12V input
1 side goes to the input of the relay, the output of the relay goes to the switched 12V input of the HU.

Wire the coil of the relay off the acc just as you'd wire in any other car. The coil should be polarity independent.

The converter should pull directly from the battery.