Please refer the following link below. It might be of some use.

technet.microsoft.com/en-us/library/cc262811.aspx

<technet.microsoft.com/en-us/library/cc262811.aspx>To the extent I
know, I will answer your questions. But I might be wrong as well.

1. Can users  continue to use MOSS normally during a stsadm  backup?

Answer: Yes, if you specify a *nositelock* switch for the backup  operation.
The above link gives details about this.

2. If not, how do users know a backup is in progress  (for example  popup
message "Backup in progress, limited  functionality") ?

Answer: Users can still continue  to view the website and its contents. But
they cannot do inserts, updates or deletes during the backup (unless
*nositelock
*switch is used, but its not  advisable as it may cause the backup to become
a corrupt one). I believe, they might get a message saying the site is
readonly or something like that, but not sure about it. Please refer the
link below for details.

technet.microsoft.com/en-us/library/cc263441.aspx

3. If a user  have documents  open before backup commence and try to save
during backup, will that work?

Answer: Yes, I believe, if you use *nositelock *switch as mentioned in the
above link. But this will make the backup corrupt.