TARGET_OBJECTS is just the list of objects that CMake handles the builds for itself. Support for adding custom objects into that genex results would be a new feature request (and would probably require a policy).
However, there are usually better ways of doing this (from a cross-platform point of view at least). VTK has vtkEncodeString to do this. If you’re using Qt, embedding via .qrc file would also be more cross-platform. This usecase has also been discussed here before.