There are multiple built-in (Cisco-provided) signatures for this:
11233.0 - SSH Over Non-standard Ports (SSH Over Web Ports)
11233.1 - SSH Over Non-standard Ports (SSH Over HTTP Proxy)
11233.2 - SSH Over Non-standard Ports (SSH Over Socks)
11233.3 - SSH Over Non-standard Ports (SSH Over Non-SSH Ports)
The latter (11233.3) appears to most closely match what you describe.