An implantable medical device includes ventricular and atrial portions, and a flexible leadlet that extends therebetween. An open channel of the atrial portion, formed along a core thereof, is sized to receive the leadlet therein, when the leadlet is folded over on itself. An interventional medical system includes the device and a delivery tool; a tubular sidewall of the tool defines a lumen and has a tether extending therein. A slot formed in the sidewall extends proximally from an open end thereof, coincident with a distal opening of the lumen. When the atrial portion is contained within the lumen, a segment of the leadlet extends alongside the atrial portion; another segment of the leadlet, being folded over on itself, proximal to the atrial portion, has the tether engaged therewith. The slot may allow passage of the leadlet therethrough, when the atrial portion is positioned for deployment through the distal opening.