The book, "Pactical Electrical Equipment and Installations in Hazardous Areas", 2005, has a chapter "Fault Finding and Testing", with a diagram showing a multimeter with its probes on either 'end' of an Intrinsically Safe barrier/isolator, one probe on the safe end, the other probe on the hazardous end. The diagram can be seen here:
http://i40.tinypic.com/334kuvs.jpg

A large X is drawn through the illustration with the text saying, "Figure 15.7 shows a meter bridging the interface. This should not be done under any circumstance with the hazardous area wiring connected." But there's no explanation for why the warning.

What are the implications of doing putting a meter across the barrier, particularly with an I/S rated multimeter?

 

IS barriers limit the energy available to a device in a hazardous area such that under fail conditions, there is insufficient energy to cause an ignition. If you bridge the IS barrier like that you are removing this limiting function by providing a path around the barrier.

IS multimeters are designed and tested to show that they cannot generate enough energy to cause an ignition. They don't provide barrier functionality.

 

draw the circuit with the meter across the barrier.

putting a meter across a barrier is akin to putting a short across it, bypassing the barrier. you just effectively removed the barrier so the circuit is no longer IS.

whether you are using an IS meter or not is irrelevant.