What makes this sign particularly noteworthy is that this is not a JR station, and the information it gives has been true for many many years. It's therefore highly unlikely that this was put up in any kind of official capacity. What I think is more likely is that he had occasion to pass by the station, noticed a huge, blank, temporary wall, and just got on with it. Either way, several hours of labour has clearly gone into this piece of what could only be described as street art, and I think it was time well spent.
I emailed Mr Sato to enquire as to this. If he honours me with a response, I will report back.
Got a nice detailed mail back from a Mr Takahashi on Sato's behalf. Apparently the construction company responsible for the renovations of Keikyu-Kamata station directly requested that he make a sign for them. As it wasn't strictly speaking his job, this meant heading down on his day off and spending a full working day on it. He then returned the following weekend, again on request, to add more lettering and generally make it nicer. Apparently he's very surprised at all the positive media attention he's been getting, as his motivation comes purely from a genuine desire to perform a useful public service. This fits well with my own general opinion that art should embrace utility.
One minor errata. Apparently he doesn't work directly for JR. He's actually a contractual guard hired by construction companies to assist in directing the public around works. In other words, one of the people you'd normally see swinging a lightsaber. He's currently assigned to reconstruction works at JR Nippori, and before that JR Shinjuku, hence the misunderstanding.