I beleive version 10 has the Synthetic Backup option, this sounds like the option you require.
That aside, you may be able to schedule things a bit better.
For example, we have one site where we backup user data files overnight, and then the next morning, use a second tape to backup C:, System State and the Exchange Store, that way all files in use during the day by the users are backed up at night.
The exchange store of course, can always be backed up whilst in use.
The other option is to purchase the Advanced Open File option, so you can backup the data files during the day.
The issues you may see with this involve its affect on server performance, and the reliability of the Open File Option itself, along with whether the datafiles the users have open can actually be reliably backed up this way (eg proprietry databases should not be backed up this way).