Hmm .. nope. Just putting it into msx.zip didn't work by itself, for me at least.
Once you have the disk.rom with the correct CRC (stored wherever), you need to go into the GUI's MSX Picker tab, hold down Ctrl and select the disk.rom and whatever .dsk you want to load (if the ROM is not listed - perhaps because you stored it in the bios directory - then you can go into the Device View tab and browse to insert it in the first cartridge slot, which should then make it show up in the Picker tab). Then, when you hit Enter - with .rom & .dsk selected - it boots the MSX with the ROM present (requiring you to input the date) and, if it's a bootable .dsk, boots from the disk. If it isn't bootable, it comes up with the BASIC prompt with an additional message indicating Disk BASIC 1.0 is present, allowing you to use the MSX's BASIC DOS commands.
I tried the same technique with a disk.rom with a different CRC and didn't have much luck. Once you can calculate the CRC of files, tho, it's not too hard to trawl the MSX sites to find the corresponding .rom suitable for use with MESS ....