Chris Harmon (of Newer Technology Tech Support) replied to this and claimed that the problem is not specific to the Newer accelerator nor the 1400. Instead, it is simply a case of the more general shut down freezes that may occur on many Mac configurations (such as the RAM disk related one mentioned above).
However, I also received the following from Bryan Goldstein (of the PowerBook Memory Source): "In regards to 'restart' problems with the NUpwr 183 accelerator for the PowerBook 1400, the 'fix' is to reinstall Mac OS 7.6, as 7.6.1 seems to be causing the problem. Newer Technology Tech Support is working on a fix. The symptoms follow: When pressing restart on the PowerBook, the machine will just shutdown, and not restart. What's more? The computer will not boot-up again with the normal use of the Power key. The reset button must be used."
Update: Chris replies: "Going back to 7.6 does fix the problem. We stopped work on a fix lastweek when we found out it wasn't the NUpowr, but 7.6.1. Now it is in Apple's court."