BUT - the point of this post is how do I consistently maintain deep midtones with bright but contrary highlights on HP5 and ID11.???
Now that I've had a day to think about this, yes I'm slow, let me start by rephrasing your question a bit.
"how can I control the exposure relationship between two contrary subject sets in one shot?"
There are actually several ways but lets start with what probably won't work.
Sure, by manipulating the agitation you may be able to adjust the relationship between subjects a little but I'd say that agitation is probably better used as a fine tuning tool rather than a rough placement tool.
Im not suggesting a change, just ticking through a list and you've actually ruled it out with your developer specification, but using a Pyro developer might actually do more for you than changing agitation. Still and yet we are probably just fiddling around the edges and there are ways to do this with any developer.
More or less camera exposure is also a loser because this simply slides the subjects, in lock step, up or down the curve; it does not change the printable relationship between the contrary subject sets, only their relationship to the ends of the curve.
If you actually want the two subject sets centered on specific points in relation to black and white on paper, then there isn't much to gain or even much of a choice in placement of exposure other than in making sure the detail is available on a usable part of the film curve.
Now to what can work.
Extending or contracting the printable range, using plus or minus development or a harder or softer paper grade, can actually make a big difference. This option can significantly change where your subjects fall on the paper's curve. In the particular situation you describe, maybe shooting your HP5 at 200 or maybe even lower and then backing off on the development, could be a real fix.
Jose Villa is a good example of someone who uses a lower contrast curve, 400 speed C-41 films instead of 160, in a similar manner to what you seem to be asking for here. Jose shoots at +1, +2, or more at times and has his films developed normally, he has chosen to change curves by switching films rather than development. By using HP5 you have the option of adjusting the slope of the curve without switching films.
Manipulation of the lighting would also make a huge difference. Using the RVs as reflectors is a really good idea, if you have the ability to pick the setting. Fill flash and or reflectors (or a scrim if you need to spread the relationship) are real workable options too when you can't pick the setting.
Changing the lighting of one subject really does change the relationship to the other and I regularly find myself in the darkroom asking myself why I didn't use a flash here. Truly and seriously I always have several strobes in my bag. If guys like Galen Rowell could use fill flash on the side of a cliff dangling from a rope, and use it well; I ought to be able to do it on flat ground. I do get lazy on occasion though.
The last option I'll leave you with is filters. Yellow, red, polarizing, or whatever, they truly change what the film can see and where certain color tones will fall on the curve. These are easy and effective in many situations, my biggest problem is forgetting them.