Altered my safety and control wiring a little.
Fitted a safety relay - Telemecanique XPS-AS24v, this might look a little more complex but its not really. The benefit is that it has two separate output channels with contacts that are forcibly opened as well as constantly monitored internally, I have one output telling the CSMIO/Mach3 whether or not its in reset and the other channel supplying power via a contactor to my servo drives - also one N/C pair on this contactor is monitored by the unit in case it does not close or jams etc.
These relays seem much cheaper than the PILZ units shown in the CS-Labs book but have pretty similar functions.
I think that's about as safe as I can get it - if a drive runs amok, it hit a limit and kill power, same with Mach/CSMIO, the e-stop/limit chain is monitored for shorts etc, the only scenario where it gets shirty would be a runaway drive with the control contactor fused in the shut position but at this low power level thats pretty rare i think.
Of course, thinking of safety as we all do all of the time

I have just created an autonomous machine (runs from code) with open moving parts (cutters etc) that has absolutely no guarding or barriers anywhere

Only mentioning this as it made me laugh when i thought of how many of the rules I have to obey all day in the print factory where i work have been broken, smashed, severely infringed or otherwise ignored
