I don't believe that this will still work. The hack relies on a hacked version of the flash-plugin!
From the TenFourFox TechNotes :
Because of increasingly poor plugin support on Tiger and PowerPC, plugins ship disabled in 10.4Fx 6.0, and no longer function at all in 19.0 and up. This is both for reasons of security (particularly with Flash) and the use of incompatible later OS X APIs.
Couldn't you use a User Agent Switcher with TenFourFox. That way it identifies itself as an iPad or iPhone and Youtube loads Flash free in HTML 5 instead. I haven't used PPC for a while and don't feel like dragging out one of my PPC devices to test it out.