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.