I think the best would be not 10% transparency but per-layer outline mode. Unfortunately this is not possible just yet.
That wouldn't really work for tracing, where you'd want the bitmap to be lower contrast. Outline mode would just show the bitmap as a square.
What perhaps needs to happen is a way to make inkscape force layer style sheets as a fundamental low level display parameter which is code-able to form such display effects that are being proposed here.
The outline mode as described would require a style sheet parameter which forces cascade to override later values, you can do with with css params such as !important but call it !allthewaydown or some such and while this data would never be saved to file it would be a useful directive to be able to control the display from within the data instead of setting up use one control logic in code.
All in all I don't think what I want to see is complex as such, perhaps what is difficult is explaining what is intended and then the guys with more experience with the code base can suggest better solutions then my above attempt.
Best Regards, Martin Owens