I suspect it might not even be the vendors, Barry - I would try and hunt down the distributor/wholeseller.
You see, a lot of these vendors are actually selling the identical thing, just under various model names. The clevo units all come off the same production line somewhere across the pacific, possibly sent semi-assembled with the vendors completing final assembly based on customer options.
Could be that whoever the vendors are getting these units from doesn't want them being evaluated.
Could also be, based on the $3k-$6k price, that these vendors might have so tight a budget or operate so small that they simply do not commit to a unit before payment has been authorized. However, if that were the case, then I am amazed that they are still in business.
THG has tested computers from Falcon NW for example, who offers the same clevo laptop as Sager, Hypersonic, xVx, Alienware, Voodoo etc... Falcon NW do not seem like a company tight on budget. It would be interesting to ask for their response.
Dell's recent purchase of Alienware might also suggest it's m7700 line of clevo units being rebranded in the not-too-distant-future under yet another XPS incarnation/designation. If/when that happens, you can be pretty much sure that the infamous clevo unit has met at least Dell's specifications, which is saying something.
Quite frankly, if these vendors have the technical apparatus to back a 3-5 year warranty on their product, the advertising finances to create/maintain those snazzy websites, and the available manpower to host a live webchat in some cases, then I see no excuse as to why each vendor should not have the resources to offer a demo unit for tech reviews.