[Musicxml export] - Flexible durations should be made in polymetric music

The attached is an example of polymetric usic. The exported Musicxml can be translated into braille, but it has wrong mathematic duration, and will cause problem when importing to any notation software. The divisions are the same, but the actual durations of eighths in every staff are different. Should we implement this as this:
1. Take 12 as the basic division;
2. Scale down or up the durations in other staves, thus 6 of an eighth for the first, 4 for the second, and 9 for the third.
It's a bit complicated, but Musescore can report beat locations correctly, I believe it can also calculate the actual durations correctly in Musicxml.

Haipeng