[Fyron]

I may be just me but it appears the lion's share of maintenance come from my mineral pool.

This is because maintenance is a percent of each resource. With highly biased mineral costs, maintenance is biased towards minerals.

...maintenance cost to be paid primarily in radioactives...

You can not do this directly with maintenance, since there is no way to control maintenance of a particular resource (other than build costs, which obviously is more than just maintenance). However, you could add this ability to engines:

Generate Points Radioactives

Give it a negative value, and presto, engines now have higher rad "maintenance" costs. This ability is not affected by any factors (racial bonuses to production or maintenance, system abilities, etc.), so it gives a fixed upkeep cost.

Just be aware that there is a bug where the points generated (destroyed) by these abilities do not show up in the empire status screen, but they do still subtract/add resources each turn.