You have over anylized this Tyler. Rod is used for expansion joints in large installation like a shopping mall where there is a gap and tile picks up again on the other side.
In you bathroom you leave the gap at the wall and will put base board over it. At the tub I wouldn't leave a gap that was more than 3/16". You can fill all of these gaps with caulk that matches the caulk.
If you want rod you need to find a tile contractors supply house. Click on the TYW store link above and ask them if they can supply it. Master Wholesale, www. masterwholesale.com in Seattle can mail it to you. If you call them make sure to tell them that Jerry The Tile Guy sent you their way. (Hey guys see how is smiled at the robots/crawlers again?