I all ready thought about that using earth magnet, as I have had some experience in that department with converting my Games Workshop Tyranid army, did my Carnifix and the latest Trygon, model, my experience is they ok with plastic as there is no weight in plastic, hence the pulling force will hold the parts together with out gravity making parts droop down, also did my Hive Tyrant that’s made out of pewter but had to have a pin system to stop gravity taking over and making the arms droop as will as the magnets.
If you want a strong grip then you need be using 5mm or 6mm if you can fit them in both arm and body.
Here is some picture of my hive tyrant with magnets on it and Trygon, the hive tyrant upper and lower body have 5mm earth magnets in them you pick hole figure up by upper body.

