Conditional tags using wordpress tags as arguments

OK, forgive me if I’m using the terminology wrong.

I would like to put something in a template so that I can call one function, or have a function that has a variable, and if that function or that variable is not present, it calls another wordpress tag.

Specifically, I would like to use this to override the_permalink on select instances and replace it with a custom tag with a different permalink (the different permalink will be the original permalink of a reprint article–please don’t ask me to explain this; the last time I asked about this the post got hijacked by someone who didn’t understand reprint licensing and basically was asking if I was a plagiarist–if you want to know about reprints go to creativecommons.org).