The -s flag (SplitStereo) isn't used in mxfsplit, the -w flag (SplitWave) however is. 
Apart from that, the header in an AVID WAV file contains some extra chunks which may prevent the essence from being recognized. (I'd have to check how this translates into a WAV file wrapped inside an mxf file, but I don't have a suitable mxf file available)
An optional solution could be to prepend a WAV header to the RAW PCM essence yourself.
The structure is very simple:
Code:
      fourcc      fRIFF;            // 0
      LEUInt32   RIFF_len;         // 4
      fourcc      fWAVE;            // 8
      fourcc      ffmt_;            // 12
      LEUInt32   fmt__len;         // 16
      LEUInt16   format;            // 20
      LEUInt16   nchannels;         // 22
      LEUInt32   samplespersec;      // 24
      LEUInt32   avgbps;            // 28
      LEUInt16   blockalign;         // 32
      LEUInt16   bitspersample;      // 34
      fourcc      data;            // 36
      LEUInt32   data_len;         // 40
                              // 44 
and the code to fill out the fields can be copied fom the mxfsplit application.
Code:
void WriteWaveHeader(FileHandle File, Int16 Channels, UInt32 SamplesPerSec, UInt16 BitsPerSample, UInt32 DataSize /*=0*/)