Yes, in this context, sus is similar to sur. The word sus still exists and originally meant en haut (up). (go to third tab SUS, adv.).

Originally, the forms desur, desor et desus coexisted until desus took off and ultimately became dessus. I assume it was easier to say. Note however that sur took precedence over sus an sor, so that sus, even if it still exists, has lost its meaning as a preposition.

Sus (see the etymology section) has been used with the meaning of “sur”, but it is no more in common usage excepted in some expressions (courir sus à l’ennemi, en sus) and even these are perceived as old fashioned (at least by me).

The word “sus” is not often used with the meaning of “dessus” anymore. The only occurrences you would probably find nowadays would appear in “sus-cité”, which means “cited above”, “cité plus haut”, “cité au-dessus”. (Not to be confused with “sucité”).