I don't know if testing them on their side would work but one I'd like to see is the Magnepan 20.7i. It doesn't matter to me who tests it so long as the methodology is sound.
The Quad ESL-57, which someone also mentioned, is another curiosity point. The trouble with those, however, is that they have to be restored (likely rather recently) in order to perform well — from what I've read at least. The quality of the restoration may vary. I have read the claim (and it doesn't seem to be controversial) that the speakers, even if new and unopened, would be degraded and would have to be restored. Perhaps it has to do with the thinness of the mylar and/or the coating used. I have read a number of times, the claim that a speaker like the Acoustat 2+2 does not have the same degradation because it uses a thicker diaphragm and a more robust coating. For those, one of the people who worked for that company said the transformers are the weak point (although the speakers, overall, are unusually reliable for electrostatics), at least those made prior to the 'medallion' specification.
I am very curious about the Acoustat 2+2, on the basis of it being rather inexpensive these days — with the reputation of being reliable if not subjected to bad treatment like panels being washed in bathtubs — but even on its side it's likely too much for you to deal with. Its claimed demands on amps is also of interest, as is the claim I've read a number of times that it doesn't have the detail of some other electrostatic speakers, perhaps because of the use of the thicker diaphragm.
Amir's LRS review found that, at least in a small format, planar magnetic technology may not offer much value. Some have claimed* that planar speakers must be very large (such as a Magnepan Tympani IV) to really shine. It's too bad testing that is so difficult.
*I believe the claim is based on the need of planar speakers to have lot of surface area to compensate for the low mass of the diaphragm, especially when it comes to bass. It may also be related to the flat shape of the diaphragm. It has been quite some time since I read into all of this.