Maybe @TIBopenpub is worth a look (I am involved and biased). Library publisher, located at TIB, Germany's national library of science and technology. Emphasis on conference proceedings, aimed at competitive, state-of-the-art publishing. No APCs, but costs for the society. Very interested in discussing this.
@tullney Thanks for the pointer. I had vague awareness of what TIB Open was up to but didn't realize conference proceedings was a focus.
Part of the decision this society will need to make is whether they just need access to a publishing platform, or would prefer access to a platform & production labor (for inputing docs & metadata, for document preparation, further indexing, etc.). The labor has to be sourced somewhere, either internally or through a service provider. Pros & cons for each.