What is the active ingredient on the rooting hormone?
Different chemicals have different methods of deployment.
You may have to just dissolve it and soak the rhizome and hope it gets absorbed by the plant.
Also remember that cattleyas will only grow new roots from currently developing pseudobulbs, or secondary roots from existing healthy roots. An old pseudobulb or the rhizome itself is unlikely to grow new roots.
If the cattleya has missed its growing season (no more new pseudobulbs emerging), you might be able to keep the plant alive until the next growing season and let it grow roots on its own.
Superthrive might be a better overall remedy compared to a single-purpose rooting hormone.