You could use the SectionExtension - mentioned here: Create a section plane leaning against one object
Documented here: https://forge.autodesk.com/en/docs/viewer/v7/reference/Extensions/SectionExtension/
This also has a function called setSectionBox(box) which might be best for what you want. If you can figure out the extents of the room then you could pass those values to it.
Here is a blog post on getting room related information extracted to SVF: https://forge.autodesk.com/blog/new-rvt-svf-model-derivative-parameter-generates-additional-content-including-rooms-and-spaces
The other option would be to find all the objects inside the given room and hide everything else. If there is a property for each object that specifies which room they are in, then you could do it based on that using Viewer.search()