I've only seen floaters with german style armor, but it's also a time period that I'm not specifically focused on, so it's possible. I usually see the half rerebrace as attached with an articulated arm assembly that points at the top of the rerebrace.
Allan could probably make you one from the pattern for his regular 14th century arms? Then point it at the top, and strap around the bicep? Possibly pointed at top and bottom? The german style points at the top, and also points to the floating elbow cop.