Bad idea to boot Tiger? Are there any possible ill consequences from booting my 2007 MacBook Pro (runs 10.8.3) into 10.4.11? According to Apple it's supported. I just want to run Diablo 2.

I found an old backup of my PowerBook G3 and if dd works right (asr strangely refuses to do it), I'll have a partition which can boot either machine, complete with a System Folder.

But I don't want this to affect or corrupt my real filesystem.
As long as you're not using FileVault it won't be a big problem.  If you are using FileVault or encrypted drives in Mountain Lion then Tiger won't be able to read them but even then it won't cause damage as long as you don't erase them when Tiger complains that they are unreadable. 

Spotlight works differently in Mountain Lion so Tiger's spotlight and Mountain Lion's Spotlight will each have to reindex all your drives when you switch back and forth but that just takes time, it's not permanently damaging. 

The riskiest part is trying to create a dual-boot disk.