Why you need/want a 1024K version?
A couple of reasons:
First is to store large midi files for playback on my Moonsound cart,
second is that I can then map the external 1MB as the main memory mapper using MemTR, the MSX 1 games 'cracked', 'trained' and transferred to disk images, then work fine on TurboR 
The external memory 'has' to be larger than the internal (for me '512k') for this to work, as MemTR maps the largest as the main mapper
I'm sure the extra RAM will come in handy for other things too.....
First is to store large midi files for playback on my Moonsound cart,
If that midi files are larger than 800K aprox., then you need more RAM indeed.
second is that I can then map the external 1MB as the main memory mapper using MemTR, the MSX 1 games 'cracked', 'trained' and transferred to disk images, then work fine on TurboR
I'm not sure about that. If those MSX1 games are converted in the right way to disk, then they should work on internal memory. If they have bugs in the memory selection rutine, they'll probably fail in the 1024K expansion.
I could test it with the 1024K prototype to be sure.
The external memory 'has' to be larger than the internal (for me '512k') as MemTR maps the largest as the main mapper
With Nextor you don't need MemTR. Just press "4" key at boot time and bigger or equal external memory will be used.
Ah, so you can still map the MFR SD RAM as the main mapper even though it is the same size, handy!
The reason is, I downloaded loads of disk images and they mainly failed to load on my TR, however, when I used my external 1024kb memory mapper (as main mapper), they worked fine!
I can load cassette images (using LoadCAS), but they are not 'trained'/'cracked', Jet Set Willy asks for the codes etc. I need the unlimited lives in Auf Monty....
Not sure why they worked on my external (now long gone) mapper, but they did 
When running a DSK, MSX is restarted to disable subslots. In this way the BIOS can detect the MegaFlashROM SCC+ in the primary slot.
That mean that all devices in MegaFlashROM SCC+ SD will be disabled except the MegaFlashROM SCC+. So the RAM will not be available.
If you try OPFXSD v1.11 you can hold "R" (cartridge in slot 1) or "F" (cartridge in slot 2) to NOT disable subslots but disable Nextor. In that way you can run a DSK and use the RAM expansion. But the SCC will be in a subslot.
I downloaded OPFXSD 1.11 and tried following things:
1. I need to restore KERNEL.DAT, because I killed it before to disable SD interface from booting. Put KERNEL.DAT into root of SD card and booted into recovery mode with key "UP" pressed. Pressed "F4" to restore kernel. Program said something about searching and counted some HEX values for a while and then returned to main menu. So, it's unclear if it located it and flashed it or not. So would be nice to have more comments in this section. So I assumed that it didn't flash and went to 2nd step.
2. Installed MegaFlashROM SCC+SD into SLOT 1 and MegaFlashROM SCC+SD+512K in SLOT 2. Booted to MM and flashed 2 files (assumed that SCC+SD+512K in slot 2-1 and it found it there! Because we don't want to flash last working interface in SLOT1-1
:
OPFXSD KERNEL.DAT /S21/I
OPFXSD ROMDISK.DSK /S21 /ID
So, I install MegaFlashROM SCC+SD+512K in SLOT1 and reboot MSX. Nothing happened, it went directly to basic.
3. So, I decided to flash DSKROM.ROM as well to make sure that everything is updated. So I used this command:
OPFXSD DSKROM.ROM /S21 /I1
Flashed with no issues, restarted and again nothing!
4. I decided to re-flash DISK1 of SD Snatcher so the boot will be flashed again. Done!
OPFXSD SD-SNA1E.DSK /D1
Restarted again and wooow! It's booted now! So, is it good news? NO! Freezing again... But I forgot to press "R" with cartridge in SLOT1! Restarted again with "R" down and MSX freezing even earlier with a blue screen, so game not even starting yet. Same effect with "F" pressed in SLOT2.
5. So I am out of options at the moment... Another interesting thing is that SD interface is still not working, if I try to boot with "ESC" key, it's going directly to BASIC...
Earlier when I was experimenting, I used OPFXSD /U. At the time it didn't produced any results, but maybe this is the reason why SD interface is not working anymore? Even it worked after I did /U. And would be nice to know which command undo results from /U command (But as I mention earlier, /U didn't do anything for me and cartridge was booting with SD interface).
So, I am very frustrated...
I used recovery with new kerneld.dat (could you add link to it to the downloads from MSXCartridgeShop, because I believe it will help many people).
I agree. That seems the easiest way of restoring Nextor/SD interface and ROM disk.
I'll join both versions. From now on "recovey.dat" will also include the ROM disk. If someone don't want/need it, it can be erased easily from recovery menu.
Nice to hear you get it working. 