A good example would be Mas Oyama sending guys out to Thailand to see what they picked up and compete against the Thais.
There is another cool example again from Mas Oyama, that he was a black belt in judo under Kimura (considered by many to be the best judoka of all time).
He had also trained boxing (although briefly), trained Aikijutsu (a older more combative version of aikido), two styles of karate, and crosstrained in other styles &arts quite often. The oyama dojo that eventually grew into the kyokushin style, was in many ways a fighting dojo where many guys from many styles met to sparr and try stuff out.
When Kenji Kurosaki broke away from kyokushin to found the original Mejiro kickboxing gym (wich was a pioneer in the early japanese kickboxing scene in the 60-70ies, spawning several champions, including MT champions), it was as a result of a dispute between him and Oyama if they should try to raise talents from scratch "inhouse" -as kyokushin -or letting other do the grunt work and pick the best students from other schools and just add the finishing touch.
But what I was referring to was the old masters sending their students to train with other masters. For example, The founder of gojuryu was sent to china by his master on okinawa, to train kungfu. Many of the old masters studied under several of the previous generation of masters at the same time.
Early shotokan guys was sent to masters of other karate styles to gain insight into, and improve, their kata -because the masters of that style knew more! (Can anyone see that happen today?) This by the way included Gigo Funakoshi, son of Shotokans founder.
The early aikido organization actually had exchange program with, and sent students to, kodokan Judo to focus on bodythrows and newaza (not that there are any traces left). It was just common practice. People went anywhere they could to learn anything they could from whomever they could, to improve -and their teachers encouraged it.
The whole "Do not change! keep the style pure" crap would have been baffling to them. The whole shu-ha-ri (copy, adapt, break-away) concept of learning required the student to change what he had learned.
Even the worst cases of secretive styles was always more then happy to steal stuff from other styles. It was always a question of "learn from them without them learning from you".
Then organizations grew, and teaching martial arts became about the money involved, and the ego of the leaders....