The pH threshold of hydroquinone with either metol or phenidone and a little bit of sulfite is much lower than the pH threshold of hydroquinone alone. I can't imagine phenidone doing all the work without being itself oxidized. When that happens, the solution turns pink to red. If you try leaving the catechol out of Pyrocat, I doubt that the activity will be much. Same with hydroquinone and phenidone. I know that would be true for my solutions, where I use a Q:P ratio of about 40:1.
There is little we can say about a synergistic mixture by looking at the behavior of individual components. Most people will say that phenidone is not much affected by bromide. The graph of superadditivity of phenidone and hydroquinone in "Theory of the Photographic Process" shows that the combination is greatly affected by bromide content. All I'm saying is that the proof of the pudding is in the eating. You know I didn't coin that phrase. My great-great-great grandmother might have, I don't know.
I think there should be some stain, as you saw in your trial. I'm not going to say anyone will like its color. It won't hurt my feelings if no one ever uses it. I only use it as an intensifier because I have the ingredients handy. I have used PMK for the same purpose, but it's more expensive and maybe a little more toxic.
I mixed a batch of Q-P-TEA as per Patrick's formula (10g Hydroquinone, 0.2g Phenidone, 100ml TEA) and tried it on a roll of Tri-X. To be on the safe side I used a 1+30 dilution and developed the roll for 17.5 minutes at 21°C. The negatives do have a brown stain that looks rather prominent to me but then I can't compare it to negs developed with Pyro or Catechol. Grain is reduced, the negatives are sharp and do print well on grade 2.
Does anybody have dilutions/times to compare? HP5+ anyone?