The changelog entry about placement of openmpt-mpg123.dll was wrong. in_openmpt.dll needs to be in the Plugins directory, and openmpt-mpg123.dll needs to be in the directory containing winamp.exe itself (Edit: This only applies to 0.4 (and later) ). Changelog corrected in r10666.
Does placing openmpt-mpg123.dll into the winamp folder work for you?

Where openmpt-mpg123.dll needs to be is dependent on how a particular player loads its plugins. Other winamp-compatible players may behave differently.

Thanks!, placing openmpt-mpg123.dll alongside winamp.exe loads in_libopenmpt successfully.