This is the first time you've written that this max power test occurs at Vy.
i didn't think I had to explicitly state it since I was talking about doing the power check before committing to a hover - it doesn't have to be done at Vy, it can be done at pretty much any speed, you are simply proving the engine.
With respect to (1) above, that is true. However if an engine has a problem producing power under those conditions it will be known long before then due to the way the engine is derated and the way that derating is respected by means of the MAP chart.
If you didn't pull to full take off power at any stage previously in the flight, how do you know it will produce it? Coming to the OGE hover short of power because your engine is starting to show its age or has a niggly fault is not clever, especially into a confined area.
It's just a power check - plain and simple - but you have proven the engine so you know it will perform as advertised. You are going to put yourself in a hazardous position, possibly well inside the HV curve if you are using OGE hover to get into a deep confined area - why wouldn't you want the confidence that the engine will perform as it should?
With respect to (2), there are no numbers in the POH that say if you can obtain a given vertical speed at max power and Vy then you can safely attain OGE hover performance at that density altitude. Thus a given vertical speed reading becomes the same sort of rule of thumb that the 6-8" of MP margin is. Both methods are empirical by definition.
I didn't say that at all - I was trying to point out that since you do have a lack of performance figures in the POH, you can give youself an idea of what the difference of one or 2 inches MAP makes in an OGE hover by trying it for yourself.
Go out at a weight that you know won't need take off power in an OGE hover, stabilise and check the actual MAP. Then see what rate of climb an extra inch of MAP - or 2 - gives you, it just allows you to calibrate your brain to how much or little performance you have left when you are pushing the limits of the aircraft. If you know you are only just below the max gross for an OGE hover then you will know you have to be super-careful if you have to go into a tricky confined area.
It's no different to checking the MAP in a low hover to see what your margin is before committing to a high vertical climb out of a confined area - you will effectively be doing the power check in the climb in that case if you hadn't done one before going into it.
In essence - 1. Make sure you have sufficient power available according to the POH ie you are inside the lines according to the graph at your intended gross weight, PA and temperature.
2. Confirm that the aircraft will actually produce take-off power at some stage of the flight before you put yourself in a condition where you are going to rely on it doing so.