Has anyone tried steam cleaning the dirt and grime (discoloration) from their engine bay?
Interestingly enough the engine bay in '04 pre-facelift R53 does have a clearcoat applied to the interior metal.
I discovered this only by accident when attempting to use several cleansers and chemicals to clean the stained strut tower housing.
A top coat degraded to a dull finish but left the paint unblemished.

I figured maybe a steam cleaner might work?

 

I'd be cautious about cleaning under the hood of the early cars. When I first bought my '02 I went to wash under the hood using Simple Green or Fantastik or some such & a soft sponge. The red paint on the strut towers was immediately washed off, exposing areas of grey primer.
The lack of a clear coat should have nothing to do with the adhesion of the red paint to the grey primer to the metal body - it's just a very, very poor quality BMW/MINI paint job.
I suspect a steam cleaner will strip it in seconds.

 

I've seen great results, but it's definitely tedious. When I had longer garage workdays I'd set up my phone for timelapse video...because why not. I have an iffy quality video of an engine bay steam clean. It's not great, but you can definitely see the color change on the strut towers.

Have lots of rags on hand!

 

+1 on the strut tower paint. I used a degreaser and it took the paint right down to primer.