Mystery bb creak

Can generate the sound when not moving by pushing down gently on pedals with brakes clamped.

Both brakes? Just front? Just rear?

Are you pushing on the pedals with your hands to generate the noise or are you astride the bike pushing with feet?

If you are astride the bike I don't think you can eliminate it being bar/stem/headset related as you could be weighting or pulling on the bars as you apply pressure to the pedals.

Frame has a threaded bb right? Eliminates the usual creak suspect, a press fit bb.

I see you say you've tried replacing most of the drivetrain, can you chuck another crankset in it and see if you can replicate, or put that crankset on another bike and see if it creaks? Doesn't need to be very functional, just enough to spin round the block and try replicate the creak.

Ta. I can reproduce the sound using both brakes, just front/rear, pushing on pedals with feet or hands, holding/not holding bars (gripping the top tube). Yup, threaded bb. I'm gonna check the bb sleeve first and if that doesn't fix it, try the cranks swap as you suggest.

I can reproduce the sound...holding/not holding bars (gripping the top tube).

I'd say this probably does eliminate headset/bars/stem then.

I've never tracked a creak down to the plastic sleeve and I'm struggling to see why/how it could be the source and wonder if those that are reporting that removal of the sleeve has solved their creak haven't unintentionally done something else (like clean out a piece of grit or remove a burr from the bb shell) in the process of removing the sleeve - but I am ready and willing to be proven wrong.

I thought I'd cracked my frame a while back, turned out it was just my shite Miche Primato cranks cracking. In that case it was a tiny hairline crack that I probably only identified because of the thread on here about Miche cranks cracking so get googling and see if there's any reports of failures of the crankset you have as that could give you an idea where to start/focus your inspection.