Well, I'm not him, but I have a suggestion or two.
On the low, cheap end, the Cold Steel tomahawks are good cheap beaters, and would be good testers. Condor Knife and Tool makes a tomahawk and a variety of hand axes, and would be interesting because there aren't many reviews on em.
There is also the SOG Fusion Tomahawk, which doesn't have a sterling rep, but might be tougher than thought previously with the new handles.
Don't know much about the high enders, but there is the American Tomahawks various modern hatchets, the Bobby Branton tomahawks, and the Ranger Knives Renegade and Entry Hawk.
On another note, have you considered doing full tests on shorter knives, like the various Scandi knives, like the ones on
Ragweed Forge?
On, and on Ragnar's site, there are some hard to find tomahawks, from the Allan Foundry and Slasinski Foundry. They might also be good testers. I have the A-23V Bearded Axe, and it is a fun little piece, but I haven't used it very much.