so i struggled for a few days to make this work, but came up with a novel solution
if your 2 rigs match quite well, then it s just a case of making sure that the rotations match reasonably well.
BUT the problem comes when the 2 rigs are pretty different. maybe the forearm is much longer or shorter for example. maybe you have tiny legs in 1 rig etc
My solution was to match the rigs on frame 0 and keyframe the resulting pose . I even use scale along normals to match the 2 rigs perfectly
then I use retarget from 'current' rather than 'rest'. This does seem to work and would like a second opinion or even see if it works for you
(I have been retargeting the Rigify rig, not the meta rig )
the one main bug was that after I stashed the existing rigify animation, the arm and leg rotations were wrong, so I had to recopy the correct pose, then it worked perfectly
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Not sure if it s best just to snap the rig the the imported rig in edit mode or do it it pose mode on frame 1
anyway you can see what I have done here with my chibli iron man
https://www.youtube.com/watch?v=WntzGaKlW8U
if I take it back to frame 1 , both rigs more or less match up