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I believe you may have understood how the constant charge concept is realized. It is the diaphragm itself that must be given a uniform, very high resistance coating so that the transducer can operate in constant charge mode. There were also some fine articles in the old Audio Amateur magazine in the 1970's by David Hermeyer and Roger Sanders on DIY electrostatic loudspeakers and direct-drive amplifiers. They included useful tips such as how to create the necessary high resistance diaphragm coating, and the needed for common-mode feedback in the direct-drive amplifier. I don't know how to get the Hermeyer articles except by buying a used copy of the Audio Amateur issue, or by buying the "The Audio Amateur on CD-ROM: 1970-79" CD-ROM. Roger Sanders has made his old articles available here: http://www.sanderssoundsystems.com/audio-related-articles

For those who don't know this, the seminal article on electrostatic driver design, including why constant charge modes results in low distortion, was written by P. J. Walker of Acoustical Manufacturing (QUAD) fame. It was published in two parts in Wireless World in May and June 1955 under the title "Wide Range Electrostatic Loudspeakers." Several versions of this may be found online; one is: http://www.onethingaudio.net/FOR/QUA/GEN/PDF/9512-QUA-GEN-HIS-WW-PW1-WL.pdf