If you know the stack layout, you don't need to do the pack/unpack. You can define the struct to match the stack layout and pass a pointer into the stack instead.
The struct may have holes that contain variables that aren't used by the nested function, but that doesn't matter. Also, you would have to make sure to flush values from registers onto the stack, and, depending on the ABI, would have to explicitly push arguments passed in registers onto the stack (that would have the struct contain a return address, but that's not a problem, either), if you need them to pass arguments to the nested function.
The struct may have holes that contain variables that aren't used by the nested function, but that doesn't matter. Also, you would have to make sure to flush values from registers onto the stack, and, depending on the ABI, would have to explicitly push arguments passed in registers onto the stack (that would have the struct contain a return address, but that's not a problem, either), if you need them to pass arguments to the nested function.