You look to be on the right track to me with your last set of questions.
I would suggest first and foremost that you scale the position but not nessecarily the size of the HUD elements. That way when you rotate your device or simply change the resolution the elements look the same but are positioned properly.
Next up I would take a look at how many rendering iPhone/iPad games actually support horizontal and vertical rotation modes. I would say hands down they almost all support only one or the other and additionally, rarely rotate to even the alternate (180 degrees) positioning that would keep the same resolution.
Lastly, to address the quality issue, this should kind of go away if you only scale positional information. What may come up however is visual clarity. And while this can lead back to quality I am just attempting to focus on the fact that if they scale the resolution very high, the HUD elements may appear so small as to be unreadable. Alternately if they scale the resolution down alot and then you can see nothing but the HUD. My suggestion for solutions to this is to develop a more advanced set of HUD elements since it sounds like you are doing one quad per element. If you could make a button up out of 9 background pieces and a text rendering on top of it it may take care of most of your issues as then you could scale the size of the hud or offer hud sizing options to the end user. You break apart the button's background into the top left corner the top bar the top right corner the left border the central background the right border the bottom left corner the central bottom border and finally the bottom right corner. In this manner you scale the size of the object (and likely the text) but not the texture, you let the corners remain the same size and the borders be the same height for horizontal ones and width for vertical ones so the texture can repeat along the edges. Same for the background section which I would suggest even just making a solid color.
Anywho, like I said you look to be in the right direction.
Hope this helps.