Yes, a plug and play memory mapper is what I'm looking for right now. As I said, the Unimapper seems simple enough to build on my own.
But I'm a bit lost when it comes to what type to choose.
I've tried some games in OpenMSX with the Popolon Sound Cartridge - It's a replacement of the SD Snatcher / Snatcher but instead of 64kb RAM it has more. With this cart I can boot the Original Disk version of SD Snatcher, but not the Melancholia-patched one (since it doesn't detect the extra ram on the Sound Cart). Correct me if I'm wrong, but that is because the SD Snatcher-cart uses 64kb RAM with Konami SCC-mapper + SCC-soundchip. (These are two different things, right?) So the Unimapper in Konami SCC-mode will probably only get me the extra RAM in those few games that use it, but not the extra sound-features?
I've done som more testing with OpenMSX + Popolon Musical Memory Mapper - that one doesn't have support for SCC, but it gives me a lot of extra RAM right away and I'm able to boot games like the patched version of SD Snatcher, Metal Gear 2 and Aleste straight from disk. So the memory mapper that one uses is probably the one I want. It should be ASCII16, right?
I suposse that you want to use your SVI, but probably the cheapest way to play most games using "original" machine is FPGA. Now they are a little more expensive that a few years ago, but you still have SX1 Mini+ for 150€ and it's a full featured MSX2+ up to 4Mb RAM, double SCC, FM, SD card reader and 2 cartridge slots.
Memory mapper is almost useless for your configuration because MSX-DOS2/Nextor is not necessary to use a floppy disk or Gotek. MSX-Music is not indispensable.
So the 512kB sound cartridge is enough if you don't want to spend too much. Otherwise, take a multi-function cartridge. It will cost you twice as much but you will be able to access almost all the software for MSX1. You will not be limited by the floppy disk/DSK size.