When a fossil repository has been opened from the root directory ("/"), the attempt to "fossil add" a file results in:
 
 fossil: file outside of checkout tree: <filename>

Trying to add a whole subdir additionally prints out a SQL error message:
 
 fossil: SQLITE_BUSY: statement aborts at 2: [ROLLBACK] cannot rollback transaction - SQL statements in progress

But once a file has been added (e.g. from a "mount -obind" of / to a subdir), it can be committed from / just fine.