If you have a trelliscope display running on a server, then you certainly can point to a trelliscope display from another shiny app by its URL. Note that trelliscope URLs retain state, so that you can point to a particular state of a trelliscope display from your app, which can be useful. We actually do this often for hierarchical trelliscope displays, where one trelliscope display is a more subdivided version of a another trelliscope display, and a cognostic href is made from the higher-level display to point to the more detailed plots in the lower-level display.
As for embedding trelliscope inside another shiny app, I think this would be difficult. Trelliscope is designed to be a single-page, full-page application, so it would be difficult to embed inside another app. Not impossible, but would require some work and some consideration for how useful / widely used it would be.
Ryan