Also, in both video versions that are generally avilable, Cronan/Tandy, Van Dyke/Moore, Weller's face does turn red. This MAY have been done in post. It's certainly possible today, but when these shows were produced it would have been much more difficult - especially in the earlier Cronan/Tandy version shot in London.

 

I'm leaning toward the pin spot, very narrow, very subtle. But I'm still open to suggestions.

Try this... It's a take on the "hold your breath", BUT you don't have to sit there holding your breath. Try quick bursts of holding your breath and straining like you're on the toilet.  Essentially your building the red face by forcing blood to your head (a little red), then delivering a couple of lines, forcing more blood to your head (a little redder), delivering more lines, and just for good measure forcing more blood to the head (that guys's head is about to explode).  Yes, it may (probably will) give you a headache but it's a lot more realistic than throwing light ,which you can't be sure is going to hit the actor properly... Half a red face with a half a red moon on the set behind the actor isn't really what you're looking for.

 

Break a leg.