John,The Bass may become its own independent build, so that it could be used to work with the ESL 57's.
If you are intending to use those large 15" bass drivers to use with your 57's the only possible speaker configuration that could keep pace with the ESL panels would be a isobaric type speaker or similar configuration design. Based on my own experience OB configuration simply will not be able to keep pace with the ESL panels. I know from bitter experience of many attempts to get a conventional sealed box sub woofer and OB bass driver to work with my 57's without success. The slower moving cones of the conventional speaker simply lagged behind the lightning fastness of the ESL panels and there was always noticeable timing issues I never resolved. You may have better luck. If you decide to go this way with the bass drivers it would be worth you researching isobaric speaker design. Other folk that have experimented with trying to get a conventional bass driver to fully integrate successfully with a ESL speaker may be able to advise to better. I know Martin Logan use a cone bass with their ESL panels but those that I have heard all suffer to some degree this timing lag issue.