What will permanently fill the hollow backs of alloy pendants?
It would be helpful if you could determine the exact type of alloy your pendants are made of, and if they are painted or coated in any way (in which case they may require sanding, chemical etching, or a primer to get the best bond). There are many multi-use products that could work, but you will achieve the best result using an adhesive made specifically for the metals in your pendants and for your application. For example, if your pendants will be exposed to water/humidity, or UV light, look for an adhesive made to withstand those conditions, or apply a protective top coat after the adhesive is dry. If you need to retain a certain degree of flex, or if you need the bond to dry clear, look for adhesives with those properties, and so on.