Is it possible to simulate ISP loader?
The parallel port would not be the problem as it's just a connector - the difficult bit would be the uC and USB, and you would need to simulate the part being programmed.
It might be possible if Atmel have a simulation program for their chips (like MPLAB SIM for PICs) but even then I'm not sure it would cover all the peripherals fully (e.g. USB) and multiple chips.   

It's a pretty simple/cheap circuit, so I would strongly suggest you just build it and debug as you go.