Apparently, mono WAV files consisting of just 1 front-left channel are supported by the encoder, but those consisting of just 1 front-right channel are not. They yield the following error: "WAVEFORMATEXTENSIBLE chunk with unsupported channel mask=0x0002" which is generated by encode.c. Since this is a valid mask, I don't see why it isn't supported.
It is easy to create such single-channel WAVs using Adobe Audition, with its Edit > Extract Channels to Mono Files option. Each WAV has the appropriate channel mask.