I modified my diagram to add your "arming" function:
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The primary issue with your design is that you've put the diodes on the high current path. My diodes are all on the low current paths so they will be small and won't have thermal design constraints to worry about.
If your light bar is high amperage (it is!) then the diodes you'd need for your circuit are quite beefy. You'll need to properly cool them because they will dissipate a lot of heat. Being in the engine compartment you've got de-rating concerns to take in to consideration. (Current-carrying components are good for the rated values only within a given temperature range. You'll have to start de-rating and upsizing the diodes to account for this in the engine compartment, so it is best to avoid these problems entirely by keeping the current through the diodes low.)
To support the arming function I added a small relay. It's current carrying capacity can be as small as you can find in an automotive relay. Make sure the relay controlling the light bar is sized for the current draw of the light bar plus some margin.
The other problem with your high-current diode solution is that the silicon diodes will drop the voltage the light bar sees by 0.7V. In a 12 V system, that's significant and results in the LED driver drawing a higher current to deliver the same power to the LEDs. Higher current always means more component stress and shorter lifetimes even if the LED driver is rated to operate at down to 11.3 VDC (ratings always assume a MTBF and not that the device can operate at one extreme of the rated values forever). This voltage drop can be reduced by using a different kind of diode, but that's still the wrong approach.
If you want to add a safety feature to help reduce the possibility that a failure in this circuit cascades to take out your headlights (or high beams) then add a 1A slow blow fuse to the "high beam" signal at the point where you tap it. No guarantees, of course... 
