Building a tool immediately so it can be used later in same CMake run

I have an interesting chicken-and-egg problem and a potential solution to it (see my posted answer), but that solution uses CMake in an unusual way. Better alternatives or comments would be welcome.

THE PROBLEM:

The simple version of the problem can be described as a single CMake project with the following characteristics:

1. One of the build targets is a command-line executable which I'll call mycomp, the source of which is in a mycompdir and making any modifications to the contents of that directory is not possible.
2. The project contains text files (I'll call them foo.my and bar.my) which need mycomp run on them to produce a set of C++ sources and headers and some CMakeLists.txt files defining libraries built from those sources.
3. Other build targets in the same project need to link against the libraries defined by those generated CMakeLists.txt files. These other targets also have sources which #include some of the generated headers.

You can think of mycomp as being something like a compiler and the text files in step 2 as some sort of source files. This presents a problem, because CMake needs the CMakeLists.txt files at configure time, but mycomp is not available until build time and therefore isn't available on the first run to create the CMakeLists.txt files early enough.

NON-ANSWER:

Normally, an ExternalProject-based superbuild arrangement would be a potential solution to this, but the above is a considerable simplification of the actual project I am dealing with and I don't have the freedom to split the build into different parts or perform other large scale restructuring work.