My first thought is this: Are there free frequencies in your area that FM Modulators, the kind that are used to inject audio into a car audio system via the tuner, so those could broadcast the signal locally, very locally?

Look up language lab parts. The place I worked for years ago, Califone, had distribution networks. I just looked. The headphones are crummy and clunky.
Do you want to provide the headphones, or let everyone please themselves with their own choice?

What is the "Composite L/R audio distribution unit" you speak of? Is "composite" ever audio?

If I couldn't find something pretty fast, I'd get several small metal chassis, some phone jacks, some wire, and some resistors. For 8 people, I'd put 100 ohms in series with each channel at each headphone jack. That keeps the impedance in a safe range. Then run it off some amplifier or other.