Oo, Melanie that's a headache isn't it! I think you're right, anything you spray on those ferns will definitely kill off the bromeliads too, as they feed so much through their leaves. You could try painting on some liquid weed killer, but there's no guarantee it won't wash off before the leaves absorb it, and get around where you didn't want it. Maybe try it on one plant, just paint the ends of the leaves and see what happens? Salty water works on some plants, not sure about ferns, but it might be worth a try.
I don't think any of the weed killers that are absorbed through the leaves will hurt your tree, though. Just be sure to protect plants below from any over-spray. (and obviously don't spray when it's windy).
Those ferns are really hard to get rid of. They make little tubers underground that sprout up into new plants just when you think you've got them all! At least the ones in the trees won't make tubers! But I would just take your pruning shears and cut off all the foliage. If you keep cutting off the leaves, the plant will die eventually. That will certainly be easier than pulling them all off that rough bark!
