Well it worked for me... at least with rev19, haven't tried rev20. I use cIOS249 base 37 with WiiFlow. I also installed RawkSD in IOS37. If v5662 of IOS37 doesn't work, try downloading an older version of IOS37 with DOP IOS.
btw, I can play RawkSD songs with RB2 loaded from both USB and DVD (backup), since both use the same IOS (249)