Thanks for those ideas, dabac. (And sorry for the typo in the original post: it should be 15mm, not 16mm -- typo!)

Grinding is likely out of the question, as I have about zero grinding skill... and the 9/16 inch is the size I need for the pedals on the mountain bike. However, I can probably replace the 14mm nuts (on front wheels of two of the bikes) with 15mm nuts -- this will drop requirement for one wrench!