The problem is that most of the heat transfer in an oven is radiant, so a transparent liquid in a transparent dish is going to get very little heat transfer. Though the air is transparent as well, it need much less heat to get to temperature than water does. One option is to use a non-transparent dish.
The emergency treatment of polycythemia (e.g., in hyperviscosity or thrombosis) is by venesection, the removal of blood from the circulation. Depending on the underlying cause, venesection may also be used on a regular basis to reduce the red blood cell count.
The goal of treating secondary polycythemia is to control its underlying cause, if possible. For example, if the cause is carbon monoxide exposure, the goal is to find the source of the carbon monoxide and fix or remove it.