Hi Chris, yes multicore solder takes on brass very well, particularly if you apply a little flux from a tin first, to help the resin core. Make surethat the metal is free from gease.If you are heating with a torch, apply the heat away from the join you are making. Let it heat by conduction, to avoid burning the flux. I think I would favour tapping the holes a suitable size, and screwing in a threaded piece of brass rod. Just thread the end of the rod, so that the thread is tapered, apply a drop of "Locktite" or similar, screw the rod firmly into the hole(s), cut off and file flush. If you do not care for chemical sealants, you could lightly countersing the tapped holes, screw in the two pieces of threaded brass rod, then run some soft solder roud the counter sink.
I hope this is some help
Regards Maurice