I've taken a closer look at this. Replacing the sram with the sdram actually wasn't too tricky. Adding OSD and keyboard also went fine and both the c64 and the 1541 seem to run as i can e.g. read the floppies error channel.
The tricky part is the sd card access. First, the 1541 wants to read 256 byte sectors which is unlike the typical 512 bytes per sector. The MiST currently doesn't support anything other than 512 bytes per sector. Actually many sd cards (especially the sdhc cards) also don't support anything other than 512 bytes per sector, so the original approach wouldn't work with many cards, anyway.
But the biggest issue is that this 1541 implementation doesn't read d64 files from a PC (fat) formatted sd card. Instead it expects the d64 images to be stored on the raw card. You'd need special tools to create such a card and a PC couldn't easily read and write this. Plus the MiST needs a proper PC/fat format to load the core file etc.
So this needs some major changes for an acceptable user experience ... but it can be done
One solution would e.g. be to store an entire d64 image in sdram and let the 1541 read it from there instead of letting the 1541 access the sd card itself in this rather brute force manner.