Is there an easy way to hide hair under a helmet without making the character completely bald?

On hair cards you could theoretically apply a filter that hides the intersecting parts of the hair-do.
But you need proper hair geometry with a flow map for that which metahumans do not offer.

If your system is modular you can’t do much.

If your systen is static, then you should author a version of each helmet with the hair-do attached and the hair that penetrates throguh removed.

Again, using the engine’s groom that’s not really going to work anyway…