I'm wondering, since the kanji rom is written double to a 27c512, would it be possible to have a different rom next to the kanji rom. For example the FM-Pak rom, so it would be possible to build in and use a YM2413 for msx-music.
What would be the best rom layout and/or what extra connections are needed to acces this rom ?
The kanji rom lives in #4000-#bfff in slot3-0. To prevent conflict with the subrom (which lives in #0000-#3fff) cs12 is used so the extra rom is only enabled for #4000-#bfff.
To use the 2nd half of the eeprom for something else some logic could probably be added to use the 2nd half in slot 3-1.
However as Nyyrikki pointed out, the kanji rom is pretty useless is Europe. I think it's possible to combine the msx-music rom and boot logo. The msx-music rom is empty from #7600 to the end and the boot logo starts at #7a00. So just copy the boot logo from the kanji rom (from #7a00 to #7fff ). However, I did not test this 
This leaves #8000-#bfff empty for something else, for instance the kun basic from the Sanyo phc-70fd2.
The 16 blocks still would have to be swapped, so the layout would be like this:
#0000-#3fff kun basic
#4000-#7fff msx-music/boot logo
#8000-#bfff kun basic
#c000-#ffff msx-music/boot logo
Another option would be to add a 3rd rom on top of the 2nd and connect this to slot 3-1, but there is no room, the diskdrive is in the way.
It should be, in theory I would recommend some testing in openmsx.
I would suggest downloading the openmsx roms.
And from this zip test with phc-70fd2_fmbasic.rom, phc-70fd2_kanjibasic.rom and phc-70fd2_basickun.rom in the systemroms\machines\sanyo\ directory.
Good blogpost luppie, interesting 
