I may be wrong, but I don't think that what you are looking for is feasible. The layer opacity is called that because it applies to the whole layer, and the only thing you can do is make part of the layer more transparent by increasing alpha, not less.

If you want to modulate higher, then you must select either 100% opacity for that layer (probably the easiest option) or the maximum opacity you intend to use and make most of your image partially transparent with the alpha channel to reach the 50% you want in most places.

On this layer you then check the layer menu to add a mask (and transfer the existing alpha channel if there is one, such as after you have gone through this process once). When you paint in the mask, black makes the painted portion of this layer totally opaque, while white makes it totally transparent. Choose your tools to put the right shade of grey where you want and it'll give you exactly the alpha channel that you want. When done, go to the layer menu again, mask -> apply the mask. You're done.

I think this would give you the type of control you want.