FWIW, I suspect the relay in the NYC Nintendo Store is no longer functional. I was poking around in 3DS system modules and found the URL
https://service.spr.app.nintendo.net/relay/0 in the BOSS (SpotPass) module, which very much looks like it was used for the StreetPass relays. This is backed up by
a comment I found on an old HomePass implementation saying that blocking that server broke StreetPass. Unfortunately, the hostname
service.spr.app.nintendo.net no longer resolves to anything.
Is the information from the
https://bitbucket.org/casperwietse/streetbasket-public/src/master/ repo mentioned in the OP available any more? I'm curious if anyone's tried reverse-engineering the 3DS-side code for the relays.