Mark,
Your ideas are basically sound. There is, however, one catch to your system.
First, as you assume, when using roll film with scenes of many different contrasts, you must develop the whole roll at one development time.
Most "place the highlights" systems are based on a "standard" development and simply let the shadows fall where they may. This is fine for normal and low contrast scenes.
The problem lies in the following scenario: Assume you have a very contrasty scene, say 7 or 8 stops between important high and low values. If you place the highlights in the "proper" place, say, Zone VIII, the important shadow values will fall too low on the film's scale, i.e., on Zone 0 or I. This means a loss of detail in the shadow areas of the negative that is impossible to recover.
If shadow detail is important, then we need to make sure that we have enough exposure to keep that detail. That means making sure we give adequate exposure.
I use the Zone System for sheet film, and expose for the shadows and develop to control contrast, but for roll film that is going to get a "standard" development time, I do the following:
1. If I'm using an averaging or center-weighted type meter ( in-camera or hand-held meter), I simply let it do its job. In low and normal contrast situations, this meter will give adequate exposure to ensure detail in the shadows. Of course, one must be on the lookout for high- and low-key situations when exposure compensation will be necessary.
However, when confronted with contrasty scenes, I know that the meter, averaging as it does, will underexpose the shadows. Therefore, I give
more exposure in this case, which seems counterintuitive at first, but ensures that there will be enough exposure in the shadows to retain detail.
When working fast, I just add one stop for contrasty scenes and two for very contrasty scenes.
Note that this puts the high values way up on Zone IX or X and, with the "standard" development the roll will get, produces a negative with a wider than normal range. Fortunately, modern films hold detail well into Zone X and higher. This then needs to be taken care of in printing by using a lower contrast grade paper. The opposite is the case with low contrast scenes developed to "standard"; they will have a shorter range and will need contrastier paper.
When shooting roll film using a hand-held or in-camera spot meter, I can either
2. place the shadows, in which case I would want to overexpose low-contrast scenes by a stop to get the shadows up off the toe of the film's characteristic curve (this is not strictly necessary although it helps shadow separation a lot). Normal and high-contrast scenes would get no exposure compensation. Or
3. place the highlights, in which case I would need to overexpose contrasty scenes by one or two stops depending on SBR (which I would measure with the spot meter). Normal and low-contrast scenes would need no compensation.
Note that all three of these methods coupled with a standard developing time for the entire roll will result in the same kind of negatives: Normal negatives for normal-contrast scenes, low-range negatives with slightly more shadow exposure than normal for low-contrast scenes, which will then be printed on higher contrast paper, and high-range negatives with adequate shadow exposure from high-contrast scenes, which will then be printed on lower contrast paper.
Just placing the highlights without some kind of system to give more exposure to higher-contrast scenes would result in shadow detail being lost. Placing shadows is often the most foolproof and least fiddley of the methods since no exposure compensation is really needed. The problem comes when one does not have a spot meter to measure shadow values with. Then the averaging or highlight-placement methods are easier.
Somewhere here (or on another forum, I'm no longer sure...) I have developed this idea in more detail. Just search on my name if you are interested in the long(er) version.
Best and good luck,
Doremus Scudder
www.DoremusScudder.com