Well yes. It would be a built-in hullmod. That is generally how special ship functions are implemented.
But the salvage gantry isn't a hull mod. It is an entire destroyer, just as it is the entire Shepard. The hull mod is a representation of the ability for the user. The ship itself is built to perform salvage, utilizing a combination of work drones, heavy space machinery and internal manufacturing capabilities which transform mostly unsalvageable parts into good ones.
To transform the ability into a hull mod is to suggest the facilities are trivial. They are not, that's why entire ships are built around them and most refitting and repairs happen at a shipyard.
Don't get me wrong. I'd love to see a large scale repair and salvage ship. However it's utility would naturally drift towards being indistinguishable from a mobile shipyard. After all you are taking useless scrap and transforming them into useful ship. That's a shipyard!