I think that this takes the original research ban a little too far. We are talking here about a place and a directly observable fact about the place. The original research here was done by the people who put the information own the wall. Any Wikipedian who cares to (I'm sure we have plenty of Wikipedians in or near Philadelphia.) can go there and take a photo of the wall for verification. It fits in fine with a policy of having people go out and take their own photos of whatever to replace fair use photos. IOW when should original research be used as grounds for rejecting a photo?
I think it's best to stop talking in terms of original research and talk about reliability of sources (which is the main reason the OR rule exists, it's a special case of WP:RS).
The source being used in the article is not the writing on the wall. The source is the few Wikipedians that have seen the writing. While the writing is probably reliable, the few Wikipedians are not. If someone took a picture, then we could use that picture as the source, and that picture *is* reliable, so everything is ok.
I think a lot of the disagreements in this thread boil down to people not understanding what the source in a particular situation is. If we only know something because a Wikipedian found it out from somewhere, then the source is the Wikipedian, not the somewhere they found it out from. A piece a information is only as reliable as the least reliable connection from the primary source to the article. If a source is published in such a way that anyone can verify it then the last connection (the author of the article) becomes reliable, as we can repeat their work (as it's a completely reliably connection, we usually ignore it and just reference the source directly).