We use this version of ECMAddAppIconMacro in ownCloud client and it works...
I tested icotool natively and while cross-compiling on linux.
SIDEBAR_ICONS are also working...
If you want to test this with the ownCloud client, it's best to use
https://github.com/dschmidt/owncloud-client/tree/fix-app-icon-macro
because that contains a small fix I just PR'ed and which is not
in master yet. (We maintain a fork of the module there, so compiling it for Windows or macOS will automatically use the version of the module that I'm submitting)