Hi, Duncan.
The short answer is: yes, you can use DisplayTitle with Cargo. The full answer requires a bit more explanation. The DisplayTitle extensions does not care how the displaytitle page property gets set. It just uses that value to replace the page title with the display title when rendering links to pages, etc. The setting of the displaytitle page property can be done with the DISPLAYTITLE magic word provided by MediaWiki core OR by the SemanticTitle extension. SemanticTitle will set the displaytitle page property with either a Cargo field OR a Semantic MediaWiki property. I recommend using DISPLAYTITLE if possible, since sometimes SemanticTitle requires an additional null edit or purge of a page before the new display title shows up. But, if using a Cargo field is more convenient, it is definitely still an option. In that case, you would use both DisplayTitle (for rendering) and SemanticTitle (for setting).
Cindy