At 7th level a magus can use Knowledge Pool to prepare any spell on the magus spell list as if it were in his spellbook.
Is there anything preventing him from turning right around and scribing said spell into his spellbook?
By RAW, nothing prevents that. It is actually a great feature in isolated situation to get new spells.
This thread discussed the issue, and has a number of FAQ hits but was never answered.
On the second page someone commented that he brought this up with Jason Bulmahn during the playtest for Ultimate Magic, and Jason said he hadn't really thought about the topic. Unfortunately I can't get to the thread where he apparently discussed it with Jason.
RAW, I see no reason why you could not; there's a possibility based on some comments in the other thread that it technically violates the "understand the spell" rule for scribing new spells into your spellbook, but I'm not certain how well that holds up.
That said, I went ahead and FAQ'ed the other thread. Since PDT seems to be hitting a bunch of older stuff from back in 2011, maybe they'll see it and respond? :)
I FAQ and Necro'd it.