Workers risk falls for your entertainment
OSHA does not have any industry-specific standards for the entertainment industry. Generally the entertainment industry would be covered under the general industry standards, although there may be some construction jobs that would be required to follow the construction standards.
What regulations apply to the riggers, truss spot operators and others in theater who regularly work at heights at the 75' to 150' range? Can a climbing belt "Swiss seat" that is used as a positioning device no longer be used as a fall protection device?
As a rigger, using a climbing belt "Swiss seat" as a positioning device may be permissible, but you can not use it as a fall protection device. A full body harness would be an appropriate fall protection device for working at 75' to 150'. As to who would be responsible for providing the full body harness, see the memorandum dated October 18, 1994 regarding the employer's obligation to pay for personal protective equipment.
OSHA is concerned with the safety and health of all workers in the entertainment industry. Although OSHA recognizes it is not appropriate to put guardrails at the edge of stages, theatrical employees need to be protected from all occupational safety and health hazards. The fall protection standards for general industry (found in Subpart D of 29 CFR at 1910.21 through 1910.32) as well as the personal protective equipment standards (found in Subpart I of 29 CFR at 1910.132 through 1910.138) are the appropriate standards for your situation.