Give the medic a primary that lets them utilize (at will by pulling out the primary- not passive) friendly projectiles for explosive jumps/escapes. This would be skill based for both the medic and the soldier/demo and would require the friendly soldier/demo to sacrifice potential damage to the enemy to help get their medic out. Not only that but it would be incredibly fun and satisfying for everyone involved.
Not sure about letting it damage enemies... one the one hand it would annoying similar to how pyro reflects are annoying. However, it's also like.. you're not negating enemy damage and reflecting it back you're just alley ooping your teammate's rocket.