Reading the 802.11-2012 spec, one could send the ANonce to the server, and the server then could generate a SNonce and construct the PTK from the secret PMK stored on the server. This would be secure because the nonces are supposed to be random. I think group key change also depends only on the PTK. This would still be more secure than open WiFi because it would not possible to decrypt packets to/from from other stations with a different SNonce.