...interventionist art

Interventionist art is concerned with either entering into a dialogue with a pre-existing art piece, or engaging with a situation outside of the art world in order to raise awareness/effect change, and often functions subversivly. It is commonly considered under the umbrella of conceptual art and often manifests itself in the form of performance.

Intervention art is fully recognised as a valid form of art, although interventions which go unsanctioned raise issues of vandalism and a discourse of the distinction between art/vandalism;

"By definition it (Interventionist art) is a challenge, or at the very least a comment, related to the earlier work or the theme of that work, or to the expectations of a particular audience, and more likely to fulfil that function to its full potential when it is unilateral, although in these instances, it is almost certain that it will be viewed by authorities as unwelcome, if not vandalism, and not art." (http://wapedia.mobi/en/Art_intervention)

Can we view Interventionist Art as being directly informed by/a product of Situationism?

I think that this possibility is distincly obvious, the fundamental core of Situationism was the construction of situations