Just in case this is useful to anyone else…
You can add functions to the end of “Common.usf” in Engine\Shaders … as in C:\Program Files\Epic Games\UE_4.15\Engine\Shaders
These functions can be called from within your custom nodes.
The editor will pick up the file change immediately, and update itself. This doesn’t require recompiling the engine.
Possibly someone can get them to add the ability to name a custom node specifically, or namespace it in some way, to avoid this exercise. The engine obfuscates the custom-node naming, into some arbitrary name.