This is why I think IP laws make no sense, arbitrary restrictions are arbitrary and stupid.
ZFS on Linux is a good example of this. ZFS and the Linux kernel are both open source, but their licenses are incompatible with one another—ie, you can't combine the code.
A common workaround for this problem is to compile the source on the user's machine. When you install ZFS in Debian, apt will automatically download the ZFS and Linux source code, recompile the kernel with ZFS included, and install the result. This is all completely invisible to the user, except for the absurdly long install time.
Describing in a blog post how to build the Frankenstein application is perhaps perfectly all right, but somewhere between there and the hypothetical application you described a fuzzy line would be crossed.
There's another fuzzy line relating to whether code is combined or not. Code that goes into the same statically linked binary is definitely combined, I presume. What about code that communicates via a REST API or a standard protocol? Perhaps it depends on the attitude of the communities that produce the code. The Linux people seem to be happy with non-free kernel modules in a way that some people wouldn't be.
I, uh, would really hope this "attitude" is standardized. GPL code should have the same meaning everywhere.
IMO, if this is an accepted practice on Linux—one of the largest GPL projects—that should be enough short of a court decision.