I had a similar situtation in a previous home. I built two wooden panels that were placed behind the speakers so that the reflection was shorter than the distance I had to have, due to other restrictions from the rear wall. I also made the panels able to be tilted slightly and found that tilting them up a bit gave the best soundstage. The wood panels were about the same size as the electrostatic panel. The whole set-up was aesthetically very ugly--which may not work for you--but it did bring the vocals back into focus.
Electrostatic positioning 1A
Here's a twist on this well-flogged subject: Bass vs. depth vs. midrange vs...How to restore the midrange balance when positioning speakers far off the front wall? The situation: when placed more than a few feet off the front wall the bass and depth are great (good news), but the vocal image becomes less palpable (ugh). Placement restrictions create only these two options. How do I restore the vocal presence that's there when the speakers are closer to the wall? Can we have it all?!