The newer Sansas don't use a directory for the database, just a file in the root; I've tried marking it as read-only and even replacing it with a 0-byte file then marking it as read-only, but the best result I got was file system corruption, with database refreshes being written regardless of any changes I made. Once or twice I've had the database refresh fail due to a lack of space, so it may be possible to pad the file system out in order to cause the database to fail, but this may make updating Rockbox or updating Rockbox's own database, etc. difficult.
EDIT: On my Clip+, the "Not enough space for Music DB. Please free 90MB" error doesn't appear until I get down to about 1MB of free space on the device. From here it's impossible to boot the Sansa firmware except in USB mode; it simply shows the error then shuts down. This may be useful for anyone compiling their own Rockbox builds who doesn't want to see the refresh every time. As mentioned, though, this imposes a pretty harsh limit on any additional content, whether compiling with a new plugin or, e.g., writing the Rockbox database for the first time, so you'll have to make sure you've got all your major space-takers done before padding the drive. Still, it's not too hard to remove your pad file, copy across your stuff, adjust the size of your padder and continue; it'd also be necessary to remove the pad file any time the bootloader needs updating.