This is a very interesting and long discussion about resistor combinations. The problem of coming up with a value quickly by combining other values is not new, I run into this problem all the time. This is especially true with power resistors. I can't imagine keeping every possible value of 2W and 5W resistors. Pretty soon I'd need a 10-car garage for this.
There are plenty of resistor calculators but what Danny is asking for is a "reverse resistor calculator". There is no way around trial and error here, but that's why we have computers. Every household has one! Why not let a PC do the work. I think an excel sheet application can be created to exactly that. This would work by having the user enter the values of all available resistors into a column of an excel sheet, then enter the desired value and tolerance, and a program would go through all possible R//R and R+R combinations. With more programming code, this can be extended to 3 or 4 resistors. This would be a great project.