Oral paste isn't formulated for maximum absorption into the skin. It could work to some extent, or may not work all that well, or even possibly work and too much is applied leading to possible OD issues. Also if applied in a spot the rabbit is able to groom it off and ingest it, if too much is applied then the inadvertent ingestion could lead to accidental overdose.
And I would definitely recommend against using the paste orally if it is the kind intended for large animals, as it isn't uncommon for the dosing to be inconsistent, and an incorrect amount given could lead to accidental overdose, which does happen and can lead to death or permanent disability. Some rabbit owners will want to try and use the large animal paste because it is cheaper than some of the other better types of antiparasitics available. But it isn't intended for such small animals and is a huge risk to attempt to use in treating a rabbit, just to save a few dollars.