Example with triplets.
The way to get the triplets is done by engraving the melody per 6/8, exporting xml to DAW, changing to 4/4, stretching the midi to fit the bars to 4/4, exporting to engraving program as 4/4 to ensure triplets are right, and exporting resulting midi to be imported to BIAB. This way the chords can be added to the bars at beats 1,3, and they will be in sync. Not all arrangements might work at beats 2, 4, but simple ones can be OK...

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Example with triplets

Artur