describe how the dialog you'd like to have would look like
Originally I envisioned a pop-up volume gauge, without a border, with an animated needle that reflects in real time the tanks water volume. Since though, I realized the Aircrane has a digital glass display displaying the tank volume. So I think now instead of the gauge with a needle indicator I can do it similar to what your doing for the propellants. This one became less important because the actual display is on the front panel at eye level so you can still see it and control the aircraft at the same time. But it would be nice to have a larger pop-up that you can position to maximize aircraft control while filing the tank.
I would still like to know if a border-less graphic such as a gauge with an animated needle is possible as that was my original concept and I can envision other use cases for such a thing.
Other indicators would be feedback coming from a sensor that indicates the depth that the boom or hose is submersed in the water. Another would be a click-able image of the water cannon nozzle's tilt angle that will adjust the water cannon angle.
Then there is basically the entire tank control panel that is on the center console where without autopilot it is close to impossible to manipulate and fly at the same time. The idea would be to replicate it in a dialog that would allow easy control of all the tank fill, dispersal settings and actions using the mouse. Were talking things that are normally changed using a knob or toggle switch.
I will also need one for the cargo controls, such as weight feedback and a cable or rope emergency shear button or switch. I'm sure there is some other things I am forgetting.
The idea is I need to be able to display and control via the mouse any displays or controls that need to be accessed in real time in a dialog that can be position as to be able to maintain control of the aircraft. It just so happens the Aircrane has a considerable amount of these types of activities that need monitored or activated while trying to control the aircraft at the same time.
Like you said I need to really think about how I want it laid out. I think the best way would be a graphic that looks like the real thing but with mouse click-able hot spots to both display information and execute actions.