Is my NEMA 14-30R to NEMA L6-30R adapter safe?
Your adapter is probably safe. However, I would have used a 3 conductor cable instead of a 4 conductor cable; that way there is no neutral to cap.

Another option would be to cut the plug off the end of the other piece of equipment and wire on the 14-30P.

A third option would be to wire up an 14-30 receptacle right next to the existing L6-30 receptacle, using the same circuit. As long as you are using only the kiln or the other equipment and not both at the same time, that's fine. If you really wanted to prevent both from operating at the same time, you could get a transfer switch, but that's probably overkill.