In fact, Dai Nippon Butokukai registered Aikido as one of the martial arts. Correct me if I'm wrong it should be Ueshiba mainline. Daito-ryu registered separately.
Talks about Aikido among other modern martial arts could lead to unusual conclusions.
For example this
. According to this article, grandmasters of Aikido, Judo and different Okinawa Karate branches share the same tatami in Kodokan. No wonder if sometimes they practice together and share each other.
Some moves from Seiryu
could be found in Nishio Budo that is a part of Aikido.
Looks like numerous koryu schools has the similar idea also. Two comes into my mind - Shinkage-ryu and Shishin Takuma-ryu.