Further investigations and comparisons finally lead to success. Now Japanese DIG looks very close to original. There are still slight differences in how the subtitles are positioned on the screen, but I consider it as a good enough (ScummVM is on the right):
Our code differs too much from the original Blast Text rendering routines, so to make a pixel perfect rendeting I would need to rewrite all of that code. Still I consider it to be good enough and if someone else in the future would want to tickle it a bit more, he is welcome.
Besides Japanese Kanji fixes, new code also fixes line breaks in Korean and Chinese versions of SCUMM 6+ games. I did not test it, but with old code there were cases when longer strings were not split in a proper way. As a side effect there are slight changes to versions which use Latin characters, so there could be potential regressions. File a bugreport if you will notice any chopped subtitles in FT, DIG or COMI.