Protecting free OS extension from use with proprietary OS

[pgraf at q40.de]

Hi,

I'm developing a free OS extension for networking (TCP/IP, ethernet, etc.) 
for an older homecomputer platform which is still used in a small hobbyist 
scene. My software contains large pieces of other free software, as well 
as many months of my own work. All pieces are GPL-compatible, and at the 
first sight the plain GPL looks as my favorite choice to release the 
whole.

In my hobbyist scene, there are basically three relevant operating 
systems, two of them are free, but the best one is (IMO unreasonably) held 
proprietary (even beyond the demise of its author).

When I release my OS extension, I want it to be used only for the free OS, 
but not for the proprietary one. The free OS and my free extension are not 
necessarily the same executable file (for ROM size restrictions), but as 
soon as my extension is loaded at runtime, I see the OS and my extension 
as "two modules combined into one program".

However I suspect that there could be attempts to "grab" my free OS 
extension for use with the proprietary OS, because the "commercial" circle 
has failed for many years to achieve what I have, and I know that some of 
their users are quite eagerly waiting for the functionality of my stuff. 
They could make some small modifications to my software and then say it 
was "separate" from the OS, just to avoid freeing the proprietary OS.

Now I wonder wether my intentions have enough protection by using the GPL 
or would I be better off with a special license that explicitely forbids 
runtime linkage with a proprietary OS?

Thanks for hints,
Peter