This is a combo that never would have occured to me. I've never used Diafine (although it's on my list), but it's used to push film while Rodinal is not. Despite its name, Diafine is reported to be a non-fine-grain dev. Rodinal is non-fine-grain as well.
I'm not sure if the two of these would click together. Perhaps after investing several rolls of film and globs of developer you may find the mark, but film isn't as inexpensive as it used to be; Tri-X nearing $50 for 100'!!!
My advice is to stick to something already tried.