Just found this interesting item!
Take a look.

About Rein-Bows The adjustable horse rein loop

Hmmm, those look interesting. I have never seen them. I am imagining how many times a ride I would be adjusting them for walk/canter/ giving slack for dropping of head/ shortening for 2-point over poles/fences/ riding western/ teaching young horses under saddle...

Yes! I think they look very interesting too and there seem to be some serious competitive riders using them (by looking at the advertising). If I was still living in the UK I would be asking around my old equine friends as they seem to have been released in Europe.
I have just ordered a pair from ebay. When my daughter is older and stronger to use the reins, I figure they may be invaluable.