[Vaupell]

well ofcourse manual edit the template, and the php file

And adding the normal register into the existing php file, then editing the template
where you want to use the var.

but hardly a way to go, cause when updateing the files might be updated aswell.
and not shareable if makeing a shared mod, but if its just for your own site
then manual editing is fine, templates will let you know when changed so you can
edit them when upgrading versions.

But if i where you, i would create a plugin, and run the preregister from there,
then also add a Str replace to add the var into use in the template.

its proberly to most "clean" way, plus easy to disable if theres problems.