1. Lifeline Material - If you're using a self-retracting lifeline for some sort of permanent use, you'll want to make sure you're using a stainless steel lifeline. This the best defense against the elements and resists rust. You'll definitely want to stay away from a web retractable. Here are some stainless steel retractables.
2. Housing - In a situation like this, you'll want to look for durable housing. Making sure the outside is constructed of stainless steel or extremely durable plastic, such as the Miller Falcon.
3. Inspection - If you're using a retractable in a permanent situation, you want to be extra diligent with inspections. Always make sure your retractable is working properly. Retractables exposed to the elements face far more dangers than a retractable stashed in the back seat of a pickup.
Make sure you keep these things in mind if plan on using your yo-yo in a permanent setup. If you have any questions or concerns about your equipment, let us know.