No I don't think any client can hide itself. All clients have their way to build and reply to some stanza, or not, and can be identified.
when you send a file to a contact, you also send him whene he can connect to get the file. The things that can be sent are :
- your IPs (the IPs your machine have. a 192.168.x.y, a real internet IP, depending on your network configuration). This is controlled by the ft_send_local_ips option.
- the IPs/hostnames of socks5 proxys (listed in file_transfer_proxies option or discovered on your server)
- your real IP discovered with UPnP-IGD if your router supports that.