TimeMachine disk locked after iMac bricked

I understand the usual method of restoring a Mac from an older one's Time Machine backup results in no apparent migration of anything. Can you access those backups if you connect the drive and hold an Option key when selecting the TM icon in the Mac's menu bar? That will reveal a previously unseen selection "Browse Other Backup Disks...". Try that and see if anything appears.

Unfortunately I may not be able to offer much more assistance than that because I no longer back up the only Snow Leopard Macs I happen to have around. If they should ever fail and I needed a SL system to test I'd just reinstall SL from disc or other original sources. I could certainly attempt to duplicate your installation in an effort to provide more helpful information, and I apologize for the fact but it would take time I just don't happen to have right now.

That bricked iMac had been running Snow Leopard or earlier, correct?

What model Mac are you using to restore its backups, and how are you attempting to do that?