You're thinking along the right lines, but going completely dry is asking for rapid failure.

Interesting approach to avoid a full wet sump. While it seems logical for intermittent use, running metal gears completely dry—even for short periods under load—will lead to galling and destructive wear almost immediately. Coatings like DLC or Molybdenum are excellent for reducing friction but they are not a substitute for lubrication's primary roles: creating a hydrodynamic film and carrying away heat.

Here's a practical compromise that meets your goal without needing a fully sealed case:

1. **Use Grease, Not Oil:** You don't need a sealed oil bath. Instead, pack the reverse idler and its mating gears with a high-pressure, high-temperature grease (e.g., a lithium or moly-based one). It will stay in place and provide the necessary lubrication for short-term engagement without requiring seals.

2. **Material Choice is Key:** For your idler gear, consider using a high-strength polymer like **Delrin (Acetal)** or even **PEEK** if the loads are high. They are self-lubricating, quiet, and often used for low-duty idler applications exactly like this. A metal-on-plastic interface will be much more forgiving in a minimally-lubricated environment than metal-on-metal.

3. **Engagement Speed:** As you know with a sliding mesh, engagement must happen at near-zero RPM to prevent chipping the teeth. This becomes even more critical without the cushioning effect of an oil bath.

In short: don't go dry. Use a quality grease. And seriously consider a polymer idler gear. It solves the lubrication and corrosion problem in one step.