I've experienced some issues when running .dsk games on MFRSCC+SD on Yamaha YIS503II (MSX1) and YIS503III (MSX2) computers. Those computers equipped with network modules (they were produced for the soviet schools classes and network was pretty useful, you then need FDD only on teacher's computer and can transfer all the data to/from student's computer by network) and seems like dsk boot code conflicts with that network ROM since pressing DEL key to disable network ROM on boot fixes the issues. Though, until you disable the NetROM by pressing DEL the dsk flashed game doesn't run (either hang or f.e. keeps printing 'Syntax error at 0', depends on actual dsk image). Other configurations like multirom etc are not affected by the network code.
Also, the keyboard matrix is a bit different so pressing numeric keys to select the disk is shifted - you press + key (it's right to ESC where most other computers have 1 key) to select first dsk, key 1 to select 2nd dsk and so on. Can i grab the actual matrix of YIS503 computers for inclusion in your manual? This probably means not only the numeric keys lines, all other keyboard is JCUKENG layout not QWERTY so it differs almost completely. I think to have this additional table in your manual would be very useful for russian MSX users.
And i've got some questions. As i've read the messages here, not all boot-time keypresses are given in the manual. I'd like to know if there's any key to disable temporarily booting OS from MFR, say, if i want to use it f.e. for SCC sound but need to boot from floppy, not SD and not ROM dsk. I already have been read that if you press 1 (while MFR is not flashed for dsk, btw) it forces DOS1 and when you press 4 it uses cartridge mapper as main. What other keys to press on boot are available? 
Another question - can i use cartridge combinations in multirom configuration (like flashing up to 4 simultaneous cartridges into virtual subslots in non-multirom mode), or it is not available? If it's available, what's its syntax in .opf file?
And - thanks for the great hardware piece 
I've experienced some issues when running .dsk games on MFRSCC+SD on Yamaha YIS503II (MSX1) and YIS503III (MSX2) computers. Those computers equipped with network modules (they were produced for the soviet schools classes and network was pretty useful, you then need FDD only on teacher's computer and can transfer all the data to/from student's computer by network) and seems like dsk boot code conflicts with that network ROM since pressing DEL key to disable network ROM on boot fixes the issues. Though, until you disable the NetROM by pressing DEL the dsk flashed game doesn't run (either hang or f.e. keeps printing 'Syntax error at 0', depends on actual dsk image). Other configurations like multirom etc are not affected by the network code.
Most of the internal softwares cause problems when loading disk interfaces. I.e.: You need to press DEL on a Panasonic FS-A1(F) to load Nextor.
When you load a DSK in the cartridge, it is actually a disk ROM simulating a disk interface that remaps the read operation to the DSK file loaded in ROM.
Also, the keyboard matrix is a bit different so pressing numeric keys to select the disk is shifted - you press + key (it's right to ESC where most other computers have 1 key) to select first dsk, key 1 to select 2nd dsk and so on. Can i grab the actual matrix of YIS503 computers for inclusion in your manual? This probably means not only the numeric keys lines, all other keyboard is JCUKENG layout not QWERTY so it differs almost completely. I think to have this additional table in your manual would be very useful for russian MSX users.
Thanks, I'll add the russian keyboard matrix to the manual.
Also, I could make a special russian DSKROM to check the right keys.
And i've got some questions. As i've read the messages here, not all boot-time keypresses are given in the manual. I'd like to know if there's any key to disable temporarily booting OS from MFR, say, if i want to use it f.e. for SCC sound but need to boot from floppy, not SD and not ROM dsk. I already have been read that if you press 1 (while MFR is not flashed for dsk, btw) it forces DOS1 and when you press 4 it uses cartridge mapper as main. What other keys to press on boot are available?
Probably you mean "Nextor keys". You can find a detailed description in Nextor's documentation.
If you have the cartridge in slot 1, press R to disable Nextor (SD interface and ROM disk) or F if it is in slot 2.
Another question - can i use cartridge combinations in multirom configuration (like flashing up to 4 simultaneous cartridges into virtual subslots in non-multirom mode), or it is not available? If it's available, what's its syntax in .opf file?
MegaFlashROM SCC+ SD does not support subslot simulation anymore. It was a feature of MegaFlashROM SCC+
Anyway, you can't use multiROM to load ROMs to subslots.
