Delivery tools of interventional medical systems facilitate deployment of relatively compact implantable medical devices that include sensing extensions, for example, right ventricular cardiac pacing devices that include a sensing extension for atrial sensing. An entirety of such a device is contained within a delivery tool while a distal-most portion of the tool is navigated to a target implant site; the tool is configured to expose, out from a distal opening thereof, a distal portion of the device for initial deployment, after which sensing, via a sense electrode of the aforementioned sensing extension of the device, can be evaluated without withdrawing the tool from over the remainder of the device that includes the sensing extension.