Have you not considered adding some more orange leds onto the actual tailgate itself? Really, you want them to be as high up as possible so people see them from as far back as possible, and they'd be hidden when the boot is closed which is a plus also!?
Just an idea
Yes, i am fully aware on where to mount strobes.
Before these small LED's, i had a light bar for the roof, trough the roof rack mounts.But those were old Xenon strobes. Sucked the battery dry within an hour. These little 6 watt (3watt / LED) were mounted on the trunkplate because the lightbar was blocked when the trunk was opened.
After a few years i had to get rid of the lightbar as the xeon bulbs were getting more expensive to order than the lightbar itself was worth, so the company did not bother.
Not a huge fan of fat halogen ones, and LED lightbars are ridiculously expensive.
And im only given a small funding by the local EMS to equip my vehicle. so i used maybe 300 on the windshield strobe.
The vehicle passed PVI inspection. Meaning that it was visible enough for the government.
I do still plan on adding a new roof lightbar, just for the sake of being visible from the side. Which is handy when towing a vehicle.
This will be the one i'll mount:
http://www.code3pse.com/products/product_info/Warn/Lightbars/H_DF
But i'll have to wait on EMS to fund it. But they too know that a proper lightbar is worth four times what my car is worth.
EDIT: Didnt notice your closed boot comment, These LED strobes are visible with the boot both open and closed. You can see that in some older pics of the rear on this diary

3 Watt LED's are blinding, so there is no point is adding them higher as they are visible from 100m+, and blinding at 50m.