UPDATE:
In order to format the Hard Disk 20 using a stock Mac 512K with 64K ROMs, you'll need to use the "Magic" System 2.0/4.1 as described above, with the HD20 INIT and erase the HD20 as usual. This will reformat the disk as MFS. But here's the catch, you can't actually use this or any MFS only system with an MFS formatted HD20 with 64K ROMs, because Finder 4.1 and earlier doesn't know how to handle a 20MB volume, even though it has no problem formatting it. On a 512Ke, the information the Finder needs to handle that same 20MB is already in the ROM, so Finder 4.1 and earlier work just fine in decoding it. Evidently the RAM-based HFS leaves a few things out.
So, in order to use this spacious 20MB MFS disk with 64K ROMs, you'll need to use an HFS aware system, which will treat the HD20 the same way it treats a normal 400K disk, as MFS, reading and writing MFS. That means Finder 5.0 or above. You just won't be able to format the HD20 as MFS using these systems, they will only format HFS. Then again, you only need to format it once.
In a quick test I discovered that MacServe will happily format an HD20 MFS formatted disk using its own partition map. Once the HD20 has been formatted with MacServe, you can use any System you like, including System 2.0/Finder 4.1. Evidently MacServe DOES know how to handle 20MB of MFS storage and lets the earlier Finder know how to deal with it. Whatever the case it works.
One further note about MacServe as I continue to experiment with it, using an HFS aware System with MacServe will also let you format the HD20 as MFS and then sub-partition the MFS volume with both HFS and MFS volume partitions. Using Finder 4.1 or earlier, will only let you partition one way, MFS. I haven't had the time really to work with anything much, but this configuration seems solid. I'll write this all up eventually and put it out there for others to experiment with as well.
In order to format the Hard Disk 20 using a stock Mac 512K with 64K ROMs, you'll need to use the "Magic" System 2.0/4.1 as described above, with the HD20 INIT and erase the HD20 as usual. This will reformat the disk as MFS. But here's the catch, you can't actually use this or any MFS only system with an MFS formatted HD20 with 64K ROMs, because Finder 4.1 and earlier doesn't know how to handle a 20MB volume, even though it has no problem formatting it. On a 512Ke, the information the Finder needs to handle that same 20MB is already in the ROM, so Finder 4.1 and earlier work just fine in decoding it. Evidently the RAM-based HFS leaves a few things out.
So, in order to use this spacious 20MB MFS disk with 64K ROMs, you'll need to use an HFS aware system, which will treat the HD20 the same way it treats a normal 400K disk, as MFS, reading and writing MFS. That means Finder 5.0 or above. You just won't be able to format the HD20 as MFS using these systems, they will only format HFS. Then again, you only need to format it once.
In a quick test I discovered that MacServe will happily format an HD20 MFS formatted disk using its own partition map. Once the HD20 has been formatted with MacServe, you can use any System you like, including System 2.0/Finder 4.1. Evidently MacServe DOES know how to handle 20MB of MFS storage and lets the earlier Finder know how to deal with it. Whatever the case it works.
One further note about MacServe as I continue to experiment with it, using an HFS aware System with MacServe will also let you format the HD20 as MFS and then sub-partition the MFS volume with both HFS and MFS volume partitions. Using Finder 4.1 or earlier, will only let you partition one way, MFS. I haven't had the time really to work with anything much, but this configuration seems solid. I'll write this all up eventually and put it out there for others to experiment with as well.