I have a photoshop file with many layers. I would like to put one layer on top where I can draw transparency, so that full white (or black) is rendered as 100% alpha, so it subtracts all the layers underneath. Is there a combination of blending modes, opacity settings, mask settings that can achieve this result?
Layer blending settings that turns greyscale value into transparency (subtracting lower layers)
What you'll need to do is place all of your layers into a single layer group.
From there you can add a layer mask to the actual layer group or you can place a clipping mask on the layer group.
To add the layer mask simply click Add Layer Mask at the bottom of the Layer panel.
The clipping mask is a layer that is placed above the layer group and is converted to a clipping mask by right-clicking it and selecting Create Clipping Mask (only available in CS6 and above for layer groups).