Did some more digging and found that AppShell was written up in the 1993 DevCon notes. It was to have provided a basic application framework with access to ARexx, AmigaGuide, new IPC standards, prefs, and other elements, the idea being that developers wouldn't have to implement all this stuff manually (incidentally, this also explains the cross references to hyper.library and prefs.library). Moving those functions to a shared library would also cut down on application code size.
That was the idea: we get a framework to build applications upon. In practice, however, we were left to our own devices.
For example, creating a working, consistent ARexx interface for an application is not terribly hard, but if you lack the necessary documentation (Commodore never shipped anything comparable to William S. Hawes' own ARexx manual and developer material, until it was already too late), you're stumped.
Building GUIs on top of gadtools.library was possible, and so was building your own user interface components through BOOPSI. At least the basic tools were available in Kickstart 2.0, whereas in the "dark age" preceding it you had to hand-craft every GUI right down to the look and feel of the building blocks. So, things were easier, but starting with Kickstart 2.0, you still had to hand-craft the GUI, but you no longer had to hand-craft all the building blocks yourself. Visual design tools, or frameworks for programmatically constructing GUIs were still absent then in 1990-1992. AppShell, AppBuilder/ToolMaker, etc. were needed, badly, but they never arrived.
The notes also confirm that Inovatronics was handling development of AppBuilder, explaining its absence from the Commodore server dump. They also say that Toolmaker shipped in 1992. Maybe I missed it on the Dev CD?
To the best of my knowledge, neither AppShell nor ToolMaker ended up in the hands of the paying 3rd party developers in Europe. No material pertaining to either can be found on the Amiga Developer CD (1992), nor on the more recent Amiga Developer CD compilations (1.1, 1.2 and 2.1).
ToolMaker promised to ease what at the time still required a lot of manual work: building an application from scratch, with a working, powerful user interface.
NeXTstep's "Interface builder" showed how you could do that, and the industry (Microsoft, Apple, IBM, etc.) quickly began adapting its concepts (results started materializing by the mid 1990'ies).
Had Commodore shipped even a modest tool for making this task a little bit easier, it would have resonated in the developer community at the time, on both sides of the Atlantic. As far as I can tell that event never occured. Commodore would have covered it in AmigaMail, for example, yet there is no so documentation.
Regarding that 2016 server dump: absence of material from that archive does not imply that it has been lost, or never existed in the first place