I would not have thought that there would be much of a problem incorporating another method into your practice, after all it is how these (all) MAs evolve. The mind body connection and pathways within the body are critical to internal martial arts, if you don't spend time developing them then you will always be missing something. In CMAs this is the Qigong and jibengong (basic) practices which should be continued ad infinitum.
Though i have never met or trained with Dan, Mark or anyone associated with than group if i did and found they had something that was more efficient or effective than my own training i would certainly find it a useful addition to my own training. I wouldn't really see the need to discontinue what i was currently doing, just have to work harder.
In the past many others have done the same eg i think Mas Oyama founder of Kyokushin Karate studied with Yoshida Kotaro a long time student of Sokaku Takeda.
It's not that it can't be done. Ueshiba did it successfully, but he also had the luxury of being able to train all day, every day, doing whatever he wanted for his entire life.
Think of it like you're a tennis player. You've been playing for years and you're very good. One day you come across someone who just blows you away and they do it in a manner unlike anyone else you've played or seen. They move differently, hit the ball different, etc. They offer to show you what they're doing and you take those exercises back and work on them a fwe hours a week, all the while playing 20 hours of tennis every week "the old way". You might eventually get to where you're striking the ball "kinda" like that person did, and you can see the benefits on the court, but that's about all you got.
Now contrast that with a situation where you stopped playing at all for a while and did nothing but focus on burning what that person showed you into your body so that you no longer move "the old way" anymore. You're going to get results faster and more likely, results with fewer impurities brought on by the bad habits of that old method.