I overtightened one of my turbo's coolant line bolts to the point where it snapped off so now there's only one bolt that's holding the pipe to the lim.
It ran for a while without any leaks until recently.
I was wondering if i could run the overflow reservoir without a cap to prevent pressure build up in loop so that maybe the coolant would stop leaking again?

 

Not sure about Australian models but USDM overflow bottles don't have pressure caps, so you can run it topless if you want.

 

The overflow bottle is just that, it's not a pressurized surge tank. So you can run it without a cap but that wont help your issue. The cooling system will still build pressure and with no pressure cap, you will make a huge mess and likely ruin the engine. It's time to fix the issue and check torque specs for the next go around.

 

But it looks like the overflow tank is connected with just a plain rubber hose to the top of my aftermarket radiator. it doesn't look like there's any sort of pressure valve so I don't see how it would be any less effective at relieving pressure than the main radiator cap

 

The radiator cap IS your pressure valve.

 

FCs had several overflow designs/setups over the years. The basic theme: wherever your overflow hose is coming from (thermostat neck, radiator cap, etc.), flip that cap over and if SHOULD be a pressure relief cap with a spring. If not, there's a mistake someone made in the last 30 years of the car's life.