Thanks to scoobie for pointing us to moot.it - what looks to be a clean and Squarespace-like discussion / forum module. I tried an installation in my Squarespace site. It does install, but doesn't seem optimal yet as a replacement for DISQUS or native Squarespace comments.
However there are only two small obstacles I found to a usable implementation of moot.it - (1) for forum integration a dedicated page for moot.it would be ideal, but there doesn't seem to be a way to inject the code into the body of a page - maybe there is in developer mode, but I don't have that; (2) the moot forum can be installed in the Post Blog Item Code Injection to create a comments section after blog posts similar to DISQUS / Squarespace comments, but it installs the full forum with categories and menu items rather than just a straightforward list of comments, and the login is buggy. I don't know if moot.it offers different code for simple comments - it probably does but I'm not up to speed on which code to use.
So basically moot.it is a GREAT solution to the absence of a forum module / page / block in Squarespace 6, and I hope that Squarespace can quickly implement user-friendly ways of installing it as a stand-alone forum on a page, and also separately as an alternative to DISQUS / Squarespace comments.