Howdy!

There was actually just recently a similar request on the Japanese forums, which got a response

We can already have gear repaired while it is equipped, but it would be really great if we could designate something like, “repair when damage is xx%.”
With the current system, I am having repairs done even at 99% and while I would like to keep playing while my request is up, getting repairs after just one fight is impractical.
Though it would be after patch 1.22, we are looking into implementing this type of setting. Also, while on this topic, we are also looking into making it possible to set repair requests by equipment slot. However, we cannot promise any specific timing just yet.