We are using the D7 MIGS 3rd party module now and are planning a conversion to 2nd party for next year. Merging the D7 MIGS module with the D6 Dialect MIGS module looks like the easiest approach. Letting one module reuse the other would cut down on the code. My thoughts on a D7 MIGS 2nd party module: http://drupal.org/node/1354206