A release candidate for Movable Type 4.2 was released today and I uploaded it without any problems. The new version is noticeably faster, as promised, and the layout on the plugins page is now fixed. Previously the sidebar was down around the bottom of the page somewhere.
I had been using the Simply Threaded and MT Akismet plugins in previous versions, and initially didn't move them over to 4.2. The rebuild errors made it obvious that unless I was also using the new template sets then I still needed Simply Threaded to work with my old templates.
I refreshed the templates to take advantage of any new features and ran into a few problems with 4 pages in particular: Archive Index, Main Index, Entry and Page. Three new modules seem to have been included here that I wasn't sure what to do about. Banner Header, Banner Footer, and HTML Head were blank pages after I created the templates, so instead of worrying about it too much I reinstated the previous versions of the templates that wouldn't rebuild. Once I did that I was able to rebuild without further problems.
It seems the Joomsayer plugin doesn't work with version 4.1 onwards. No button shows up on the entry screen. As a workaround, I looked at the code from a previous post, pasted it in to a new post, and changed the content and it published fine. If you're using the Rich Text editor, be sure to paste the code into the HTML screen, not the WYSIWYG screen.
TypePad AntiSpam has been incorporated into MT4.2 and is based on Tim Appnel's MT-Akismet plugin. I used this plugin in conjunction with MT Autoban which I haven't reinstalled either, but I'll wait to see how it compares to my old configuration for fighting spam. I may have to reinstall Autoban if the trackback spam gets out of hand because it has done a good job of keeping the spammers at bay.
I'm impressed with the improvements in MT's latest update and look forward to the official release. Let's hope the wait isn't too long.
Of course, your work-around will work, too, and will continue to work with the updated Joomsayer.