does your circuit have a sequential part? because it does you can use it to determine when to assign to output.
what you are saying is not useful, as far as I've understood. you are saying for example you have a wire and you want it to have no value for a time(null), and than you will assign a value to it when you want. null value is not possible in hdl. because every wire and register have a random value at reset, since they are actual bistable solid state devices.
what you can do is that for example, have a cycle counter. and say your circuit finishes its calculations after 150 clock cycles. and you can have an output register that is all zeros upto 150 cycle counts. and you can set that output after the 150. cycle. and you can have also have a one bit signal, say "done". and you set this done signal olnly after the assignment to the result register is finished. this way you can tell the next module that this part of the sytem has calculated its output value, and the value is ready to be taken from its output register.
this is the common way to do what you ask for.
hope this helps.