More assembly reading today. And some findings too. There are 64 patches, each of them is 26 bytes long (I figured it out the hard way - by reading the save/load routines, but this number and the internal patch structure is described in the service manual, d'oh!), 0x2000h is the patch #0 address as well as the beginning of ext-RAM, next free address (0x2680h) is the "working patch" address, i.e. where the currently edited patch parameters are stored. BTW, if you look at the patch structure, you will see that there's no single parameter or parameter pair which occupy 4 bits at most:

So if I'd like to add the new bank parameter, I either have to do it using two addresses (to store two 'high' bits and two 'low' ones) or to reduce the number of banks to 8. But the bright side is, that I'd be able to store the bank numbers along with the patches. Neat! I started to draw a RAM memory map, but I still have too many gaps.