That depends... One could, e.g. take fs code and write a userland implementation of the same fs derived from it, speaking 9P or e.g. FUSE. Derived as in "uses block allocator functions copied verbatim from the original". Leaving it under GPLv2, of course. Now, 9P is network-transparent. You've got yourself a server for a network filesystem, Linux boxen can connect to it and mount the sucker. *Can* we use the license to prohibit e.g. Plan 9 clients from connecting to such server? I seriously suspect that it would qualify as additional restriction (not to mention being utterly obnoxious).
Depends on the nature of API they are exposing... Hell, nevermind 9P; suppose somebody implements userland nfsd using block device and using something like libe2fs for fs access. Put that sucker under GPL; are you seriously claiming that mounting from such server makes your nfs client a derivative? Or that one is not allowed to use any kind of GPLed code in such a project?