Some initial ideas for sound approaches:

1. Performer performs to precorded audio segment.

2. Performer signals.
Performer does a designated gesture indicating that something should be turned on in post.
Ex. Ben points to his eye. This is the cue to start a drone during post.

3. Performer signals (hidden)
Performer triggers a switch unseen which marks a point in time, eg., in SMPTE. This indicates something to be added in post.

4. Performer starts audio.
Ex. Performer turns on a tape recorder. Output is split--to monitor so Ben can hear it during performance, and to multitrack deck for later mixing.

5. Motivated live performance.
Performer plays an actual instrument (e.g. poles, or percussion) that is, in peripheral way, motivated by the story. Take care that this doesn't become 'Stomp'
Ex. Performer is going out so he puts on a hat. But this hat has weird "feelers" on it. Whenever performer turns his head, the feelers hit little gamelans.So maybe, as performer looks around at various phenomena, he ends up playing a weird tune.

5. live foley
Performer triggers a sound that accompanies action
Ex. audio triggers on feet trigger samples of footsteps.Could be used to imply different spaces (e.g., sloshy footsteps).

6. Electronic instrument
Performer uses gestures/ etc., to play an electronic instrument

7. On/off ambient loop
An ambience or loop is constantly running during the performance. Certain movements make it audible. Good for defining one space from another.
Ex. Performer has foil on each foot connected by a wire in the inseam. Audio is routed through two long strips of foil on the floor. When performer steps on both strips, he connects the circuit turning on the ambience for an area. Going to different areas triggers different ambiences. Maybe the trigger would fade in/out so transitions wouldn't be so harsh.

8. The high tech equivalent of the above is to use visual, IR, or other sensors which are triggered by Ben when he enters certain spaces to start ambience.

9. Post production
Import sync events are done standard post-production style.

Note:
The idea behind these isn't to be restrictive, but to be expressive. If any of these ideas sounds restrictive, it needs to be rethought. The overall shape of the piece is most important rather than any kind of underlying system. Probably the most important general concept is that the entire stage/set is kind of instrument that the performer plays/follows/leads as required.