I haven't used omniback, but have used many Unix based products with Oracle databases (Legato Storage Manager, Veritas Netbackup, CA Alexandria, etc.)

These products in their base forms backup regular Unix files well. However, when it comes to backing up Oracle Databases, you must purchase an "extension" or "agent" type program. This agent program will allow you to either interface with RMAN or backup your files individually while the agent takes your tablespaces in and out of backup mode.

Without these "agents" (well worth the price IMHO), you have alot of scripting ahead of you. The options I can think of is:
1. Figure how to backup one file from the command line and incorporate into your hotbackup script. This is very difficult as most of these packages run off "backup jobs" (preconfigured jobs that backup filesystems).
2. Use RMAN or your hotbackup scripts to backup to disk and then kickoff a filesystem backup for the target filesystem.
3. Use a third mirror as justadba suggested.

I like the DLT 7 or 8 series drives. I think they are plenty fast enough for most database backups.

As you pointed out, you don't want to put a lot of tablespaces in backup mode if you don't have to.

Also, there would be timing issues between when the tablespace was scheduled to go into backup mode and when the backup job kicked in. And what if the tape was't done when you took your tablespaces out of backup mode?