Ah yes, that works. I tried on a different wiki post that is the only one in the thread. Happy that that works, at least, though it is yet another constraint.
I used the first post in the thread as uneditable instruction posts for editing the wiki below it. This is often useful to explain what kind of information to be collected in the wiki and how to format it, etcetera. And without ‘polluting’ the wiki contents with those instructions (wiki markdown is regularly copied to other non-forum locations).
But I accept this as a workaround. Though “bump upon edit, if the last post is a wiki” would have my preference.