Sorry for the slight abuse of the blogosphere, but I did post this request (twice) on kde-devel, and got no replies. I'm having trouble getting my custom widgets working in Qt4 designer; read on if you think you might be able to help, or if you want to learn more about how to use custom widgets in the new designer.
In Qt4 Designer, it is apparently not possible to insert custom widgets unless you have written a designer plugin library which defines your widgets. While this is a lot more overhead than what Qt3 designer required (just specify the name of the widget's header file), there is a "makekdewidgets" script available that makes it really easy to create the plugin library. However, I can't convince it to install my plugin library.
I have relocated all code defining custom widgets for KStars into a new "widgets" subdirectory, and constructed a Makefile.am that is supposed to create two libraries: libkstarswidgets.la contains the object code for the widgets themselves, and gets linked into the main application binary. kstarswidgets.la is the designer plugin library.
To create the designer plugin library, I simply followed the existing implementation in kdepim, which was pointed out to me by Volker Krause. It's actually quite simple: you just create an XML file that describes each widget, and add some fluff to the Makefile.am to run makekdewidgets on that file and compile the result.
It mostly works: I get both libraries created when I run "make". However, nothing gets installed to my Qt4 designer plugins directory when I run "make install", and I can't figure out why that is. If anyone can help me out, I'd appreciate it.