This four-year-old comment (thanks, Derek) led me to revisit Stephen C. Compton's 2003 Rekindling the Mainline: New Life Through New Churches (link for reference) to see what's still applicable and what's not. My (used) copy arrived today.
In the meantime, be sure to see my widget in the sidebar, which counts up the number of days since the last member congregation was added to the UUA. Alas, none are scheduled to join at the next UUA Board meeting, but Unitarian Universalist minister and blogger Dawn Cooley points out a report (the report, in PDF) (thanks to her) to the UUA Board that recommends lowering the required quantum of thirty charter members for admission. Fascinating. I need to give it a close read -- lots of references back to the UUA bylaws -- and will report on that soon.