Assign a CB Auto Action to one specific page?

Field: Custom > Field Value
Custom Field Value: [cb:url location="itemid" /]
Operator: Equal To
Value: MENU_ID_HERE

I assume you meant the above to be the Condition for the onBeforeCompileHead trigger.
I couldn't find Custom > Field, so I assumed that was a slip of the brain for Custom > Value.

Whatever, this didn't work.
For the avoidance of doubt, I took the menu ID from the ID column of my list of menus (Joomla > Menus > Hidden Menu).

If I could, I'd upload a screenshot, but uploads of images always seem to fail for me.

I couldn't find Custom > Field, so I assumed that was a slip of the brain for Custom > Value.

Yes.

For the avoidance of doubt, I took the menu ID from the ID column of my list of menus (Joomla > Menus > Hidden Menu).

It's not a guarantee. It just depends on what's available during that Joomla event. I've no idea what Joomla behavior has been initialized at onBeforeCompileHead exactly. I don't recall where in the rendering pipeline that event happens exactly so am unsure if routing has happened yet to allow access to routing details like the Itemid. Might be possible to extract with [get_Itemid] or [request_Itemid] to try and force it out of the URL, but probably won't work if routing hasn't reversed the URL yet.

onBeforeRender might be more reliable here since it's after routing and just before a component renders. So maybe worth giving that one a try instead. Joomla's event documentation is below, but it's relatively barebones. Everything listed there you can use in CB Auto Actions by just prefixing with "joomla_" but most if not all should be in the dropdown already.

manual.joomla.org/docs/next/building-extensions/plugins/plugin-events/

When I dig into implementing the Menu condition I'll be testing against extremely early and late trigger processes (e.g. before renderer) and putting workarounds in to handle it automatically, but for now you'll have to just use what's available.

If I could, I'd upload a screenshot, but uploads of images always seem to fail for me.

They should upload fine, but the preview will fail since we don't allow inline data URIs which Kunena is using for previews. If it fails entirely does it give any errors? If so please provide them and will let Beat know so he can take a look into it server side. Might be being caught by our security since we've security really high due to bots.

onBeforeRender might be more reliable

Using onBeforeRender as the trigger doesn't work either.

Might be possible to extract with [get_Itemid] or [request_Itemid] to try and force it out of the URL

I'm not sure what 'force it out of the URL' meant here. But, I think you might have meant 'avoid depending on the URL'.
I tried all six combinations of the 2x3 matrix below and none worked:

• Trigger: onBeforeCompileHead || onBeforeRender
• Custom Value: [cb:url location="itemid" /] || [get_Itemid] || [request_Itemid]

Can we give up now? I'll just use headtag. It took a few minutes to configure and worked straight away.

If [screenshot upload] fails entirely does it give any errors?

I click [+ Add Files], use the file selector, click [Open], then in the box beneath [+ Add Files] I just get: A .txt file uploads OK, but .png and .jpg fail (I haven't exhaustively tried all the listed file extensions). Perhaps it could be something to do with moderation falling into a black hole?