For most purposes, legibility of the "7" would be enhanced far more by adding a serif to the left side of the crossbar and emphasizing the curve on the downstroke, than by adding a dash through it. There are a few rare situations where adding a dash might improve legibility, but they would generally involve things like markings on elastic surfaces. In such contexts, putting a strong bar on the bottom of a "1" might result in its being mistaken for a "2", but a distorted "1" without a bar at the bottom might be mistaken for a distorted "7". Adding a crossbar to the seven would yield a shape which, even distorted, would not resemble the distorted form of anything else.
Unless really severe distortion is expected, however, bending the down-stroke of the seven is probably a better approach than adding a dash. Note that some fonts add a dash to the "Z" to distinguish it from "2", and adding a dash to the seven might cause it to resemble a "Z" whose bottom got cut off.