After reading this paragraph in GPL FAQ I feel confused. That paragraph says that a proprietary program can use a GPL-licensed program via aggregation - as a separate program.
Now consider an out-proc COM server. It is a separate program that is used by another program via a well-known interface. Looks pretty much like aggregation.
Can I write an out-proc COM server using GPL code, publish the server (and the server only) under GPL and use it from a proprietary program?
Well, technically an out-of-process server fits the bill, uses pipes and rpc. But this is going to give you a headache (rearranging the verbiage):
But if the semantics of the communication are intimate enough, exchanging complex internal data structures... This is a legal question, which ultimately judges will decide.
Without defining what "intimate enough" means. Reminds me of supreme justice's remark: "I can't define porn, but I know it when I see it." That ought to be relatively cheap, asking an opinion about intimate software interfaces is going to be a lot more expensive. If you use it, make sure you won't ever be wildly successful.