QWS_KEYBOARD keymap doesn't accept resource location

QWS_KEYBOARD=LinuxInput:keymap=":/keymaps/de-latin1-nodeadkeys.qmap"

doesn't seem to be a valid path for a keymap, despite the docs stating "Note that the file name can of course also be the name of a QResource.". I tried a couple of combinations of the path with and without alias, double and single quotes etc., but the only way I can load the keymap successfully is from a path in the filesystem.
My best guess would be that the colon in the environment variable is parsed incorrectly.