Aha, I figured it out. The logic used by the DMC-G2 camera for the "RESET COUNTER" appears to be:

1. Increment the group nnn (P100, P101, etc.) by 1.
2. If the photo Pnnn0001.JPG does not exist, set the photo counter to this image and stop.
3. Otherwise, increment the group nnn by 1. (Wraparound '999' -> '100')
4. If we haven't tried this group # before, go to step 2.
5. Otherwise, give up and don't reset the counter.

So to reset the counter to 200, just create directories 100_PANA, 101_PANA, 102_PANA, ... 198_PANA, 199_PANA each with one file in it.

To reset the counter to 100, create directories up through 999_PANA without any 100_PANA directory present.

I wrote a script with jsdb to do this on my Mac:

function pana_reset_help(root,firstnum,lastnum) {
  for (var i = firstnum; i <= lastnum; ++i) { 
    var d=root+'/DCIM/'+i+'_PANA'; 
    system.execute('mkdir '+d);
    // now write a dummy file 
    // to prevent the RESET COUNTER from using this dir
    var S = new Stream(d+"/P"+i+"0001.JPG","w"); 
    S.writeln(); 
    S.close(); 
  };
}

// example run: pana_reset_help('/Volumes/sd7',113,199);