If other recent observation don’t include CW singals from Z-Sat, feel free to mark the transmitter as inactive in a suggestion in DB. More details how to do that you can find them in Transmitter Suggestions - SatNOGS Wiki
After a week’s poking at the signal and getting stuck I notified @EA4GPZ and gave a strong recording to him. He worked out the mystery and made a blog post and X-link
According to the archived Chubusat-2 website the downlink of the BBS is at 9k6 GMSK.
Since telemetry is also at 9k6 GMSK and could be decoded with a G3RUH modem according to JA5BLZ, it’s likely that BBS downlink also uses G3RUH.
However, the BBS doesn’t make use of AX.25, but of a different format.
I’ve tried to uplink a message list request and to send a message using the described format in the hope that the 9k6 signal was just an empty transmission from the BBS, but without any response or change in the signal so far.
Be aware that the uplink is at 1k2 FSK not AFSK. I don’t know if it’s also G3RUH scrambled or not.