This feature request is the result of trying to create a four-sided shape in that very place, just like you indicated. 
I frequently chop part of a surface out like that to change a 3-sided opening to a 4-sided opening.
However, I have never been able to to create a curved blend to a corner (where a curve radius goes to zero)
using a four-sided shape in Rhino. In fact, in this example I have two other such blends to corners.
Even with a 3-sided shape, it takes a lot of coaxing to get the fill reasonably smooth.
Problem Corners 2.3dm (7.0 MB)
With the four-sided opening, one side does not quite come out as tangent.
It should not surprise me that the blue prints are nebulous here as well. The tangent circles at at the left are specified. The circular transition to the end is. But the rest is left up in the air.
If I try to fix that on its own I get this:
Which gave me the result for this feature request.
Another feature request might be a simple way to bring a radius to a zero.
Clearly these 3-sided shapes cause difficulties with Rhino. If I don’t join in the right order I get a bad surface, which can be anywhere in the polysurface.