Sorry for the slow reply. Thanks both for your responses! I've had a quick look at GLSL #include before — I'm not exactly sure how to use it (this is a start though) — but apart from that, I'd prefer a way to do this in Qt, possibly more customizable and also driver-independent (e.g. I'm not sure if all graphics drivers support this GLSL extension). In addition to that, it would be a good feature to share with my students who might use Qt also for later/future projects.
I'm developing the software in an academic setting — when the (to be included) GLSL files have changed, things need to be (pre)processed again. Basically I wondered whether it's possible to automatically check .glsl files with a preprocessor for some tag (#pragma seems a good choice) and act on it, in this case use some sed/regex line to replace the mentioned file by the contents of that file. This could happen before every Build, or even before every Run (depending on how straightforward it is to save the copy-pasted version after Building and re-use it when Running, or just do this before every Run).
I might be wrong but I wonder if you're not entering a corner case here. I'd recommend posting this to the interest mailing list. You'l find there Qt's developers/maintainers (this forum is more user oriented)