The org.dozer.fieldmap.MapFieldMap.MapFieldMap(FieldMap) constructor should also copy the customConverterParam property of the other org.dozer.fieldmap.FieldMap:
setCustomConverterParam(fieldMap.getCustomConverterParam());
Otherwise the org.dozer.ConfigurableCustomConverter doesn't receive a customConverterParam, if you use a mapping like this:
<field custom-converter-id="MyCustomConverter" custom-converter-param="MyParameter" >
<a key="aKey">aMap</a>
<b>destinationField</b>
</field>
fixed in trunk. Will be released in 5.2.1