There doesn't seem to be a lot of information about it and the speculation is currently that there is an additive that can be seen through spectroscopy or light, a luminescence or particle based thing which is visible.

They also mention that iSIS is made of three security types: "overt, covert and forensic".

So I don't think it's the case that luminosity is the core of the security, that is just the overt layer.

The keywords they keep using are "integrated", "embedded" and they state that the coin is "full-plate" and that they can produce blanks (for full-plating) that have iSIS protection.

They also mention (in the bottom video on this page: http://www.royalmint.com/business/circulating-coin/isis ) that unlike electro-magnetic signatures that have variability and also can vary over time (with coin wear), the iSIS has a binary yes/no response: authentic or counterfeit. It's a signature detection technique and the signature is not related to the size/weight of the coin.

Is it possible to make the interior of the coin in such a way that a radio signal bounced off of it would produce a received signal of a certain signature?

Some of the speculation has focused on patents recognised as being held by the Royal Mint that dealt with luminosity in the plate layer. I think this is just one of the security measures, but isn't actually the iSIS method.