For parts such as Space Marine backpacks, or indeed riders, that do not have a clear surface that will end up in contact with the base, I use a dab of blutac/postertack on top of my set of hobby aligator-clips-on-a-stick. Those came with a base with a grid of holes in it, so the rods can be insterd into those for easy handling/drying/storing.
I reckon you can do the same with riders; a bit of blutac under their bottom and on top of a rod, and you'll have plenty of access all around, and do not have to drill holes and pin them.
Although thorough me certainly does this with metal and resin models regardless. Plastics are the only exeption, because polystyrene cement will simply melt the parts together...