Paul,
Below is the general approach I would consider.
First I would put some packing tape over the deck area where you are going to locate the fitting. This is to protect the deck and negate the need to clean resin off the deck.
Please also note that due to the viscosity of the resin it is not always easy to fill a small hole with resin.
You will think it is filled but usually ends up with an air bubble in it that can be most of the void you are trying to fill. The smaller the dia hole the harder it is to completely fill.
Basically I would consider drilling a 1mm hole through both skins and a hole larger than the dia needed for the rivet in the outside skin but not through the inside skin. The 1mm hole left through the inner skin is to ensure air can get out as the resin in injected but small enough to retain the resin thickened and strengthened with micofibe blend or milled fibres. The larger outer hole will help when trying to scrape out as much foam as possilbe to increase the area the resin can bond to and better distribute the load. I'd probably scrape clean the top of the inner skin under the outer hole and use a vacuum to extract foam debri to ensure the resin bonds to the laminate and not to stray bits of core.
When you mix in the microfibre blend or milled fibres with the resin make it not too thick and not too thin and you really should inject the resin into the hole with a siringe.
If you try to put it in by other methods you will most likely end up with a bit of resin on top and a lot of air benieth. I've leave the resin slightly proud of the deck and trim with a razor blade once cured but still green. Then remove the packing tape.
Last I would probably not rivet but screw into the resin insert I'd made in the deck.
This is just the way I'd approach the same issue if it was my boat.
Hope this helps.
Regards,
Phill