Nebulizers can be used for bunnies with respiratory issues. One of our members used F10 antiseptic in a nebulizer for her chronic snuffles bun. (This was in the UK). Saline can be used too. Some antibiotics can be nebulized.
Rabbits can have N-acetylcystein, which helps make mucus less thick. I have only administered this orally.
One of my buns got Bricanyl (terbutaline, a broncho dilator) for pneumonia. I got pills that I divided and dissolved in water and gave in a syringe. It was only for 5 days, I dont know if it’d be good for prolonged used or if it could perhaps be given as needed. My bun got antibiots as well and meloxicam (and made a full recovery).
I think both Bricanyl and N-acetylcystein are prescription only in the USA, in the EU Bricanyl is prescription only, but NAC can be bought as a supplement. You should of course still ask your vet if they think it could be a good option for your bun.