Mounting an antenna onto a roof basket would work fairly well.. Its something I may go to soon for a couple of my rear mounts. I have not yet figured out how a roof basket will affect the nice roof ground plane.
Here are a few pictures of my installation -
Shown, a High Sierra (see below), and a couple roof rack bar mounted antennas - a Comet 2.4 Ghz and Diamond NR-770 2m/440. I have a fourth mount - a Diamond K400-UHF (barely visible over drivers side door) - much like the K400-3/8 shown below. It is used for my 2m/220/440 or 2m/6m/440. I dont have pictures of it, but thats where one could set up a UHF spot.
Picture of my roof rail mount -
This is a Diamond K400-3/8 mount. A standard K400-UHF mount works identically. In this application, the antenna mounted is a High Sierra Sidekick.
Hard to spot - there is a copper strap (plumbers tape) running from the bolt on the mount along the track to the first torx screw location; the copper strap is wrapped in heat shrink to hide it in the rail. I had to drill a hole into the mount for the strap bolt. Took some finesse to sculpture the strap through and down into the notch. In this installation I did not drop the antenna cable down a torx hole, but could have done so, likely to the one forward of this location.
There are some downsides - the antenna mount is quite exposed above the car and all manner of winds, speed and weather induced. To install it means drilling holes under the rail to access the set screws, which have loctite. And thus inserting the K400 means you cant release it or the roof rack bars behind without removing the entire roof rail assembly. With also one on the other side, this means a lifting both rails, rack bars, and mounts in one piece off the top of the car.
Looks like you guys are doing Bonnet (hood) mounts, seen some pictures of that. It is a good spot for the applications (UHF) established.
Another alternative, unexplored and mentioned earlier - using the four unused lift gate bolts - shown in picture below drivers side under taillight cavity - then building a mount that goes through the plastic bumper elements, and also using the thru-body cable feeds for the trailer hitch as suggested by Aircooled. This would work for larger tuneable antenna pipe structures like the High Sierra-1800. Probably not a great spot for the much smaller UHFs.