Damian Stewart wrote:
then there's the question of whether any and all Pd patches are 'derived works' (derived from Pd) or '[a combination of] two modules into one program' and therefore need to be GPL.
No. The GPL only applies when distributing compiled binaries, as far as I'm aware.
.pd files are as much derived from Pd as a .py file is derived from a Python interpreter or a binary from a compiler (a compiler is a tool for creating binaries, which are derived works of their source, not the compiler).
Pd's linking is at runtime, you can distribute a collection containing GPL externals. and otherly licensed stuff, because they aren't linked together until you run it.
The output from a GPL program cannot be licenced (eg, it would be a nonsense for (LGPL) GCC to force all C programs it compiles to be (L)GPL). What you can do is license a particular recording - I license my patches under GPL, and time-consuming renders under CC license (but anyone who bothered to re-render could use the output as they wished, as far as I understand).
BTW, this is from my fuzzy understanding of GPL2, v3 might be different.
Also, Pd isn't GPL.