For the problem with open files. I'd ignore what I said.
I did a test (locally) and managed to rsync my running database without any problems... and the error does say Permission denied
so I guess the problem is down to permissions.
There are some great tools for dyndns so that wouldn't be a problem.
As for the scripts behaviour, it wouldn't be a problem and you can always put fail safes.
I think the advantage of pushing to an rsync server is that your root user can do the pushing to a rsync user with limited rights. This will give you a bit more security.
We can work on your permision problem though. What do you have when you do an
# ls -ld /var/lib/mysql/vsftpd
# ls -l /var/lib/mysql/vsftpd