I think that I'm still too much of a novice to make a real contribution here, but I'll try anyway. I study the Chiang Jung Chiao lineage of Bagua, from a man who is a Xing-i expert as well. He matter of factly states that the five elements are present in Bagua. He also likes to point out the Xing-yi moves that are present in our system. The opening posture for example "lean against horse" has an often overlooked application. The arms trail behind after the push, I always took this as a transition movement, since it is more apparant in the single technique drill. When my teacher showed me the application of it I almost felt my elbow separate. (as a side note, apparently joints are capable of generating a sense of fear independent of their owners) Other moves like "Pheasants fighting" or "Lion rolls the ball" all derive from Xing-yi. However these are Xing-yi techniques that are present in a Bagua system that is rooted in some Xing-yi. More in line with your question I guess would be an observation I've heard from a really cool Tai chi man. "Once you learn Tai Chi, your boxing will become Tai chi"(He was previously a boxer). As our internal skills grow and develop, that application bleeds into other methods of fighting that are natural to us already. I'm not really familiar with the 5 elements of Xing-yi at least in those terms yet, but I've heard many "hard" stylists who converted to a "soft" style who say that their Shaolin/Hung ga/Western boxing became softer, and more adhering. I hope that I've been able to contribute something that you were looking for.
All great truths begin as heresy