0
apart from the fact that the ARU has to approve all international contracts. If they're happy that the Force can't find a Wallaby eligible scrumhalf they'll let Alby stay longer (happened with Daniel Braid) and if they think Sias is worth the trouble of qualifying for the Wallabies they'll jump to sign him up.
If the boks are looking weak at 10 watch the ARU rush to sign Sias to his third year and get him on the pitch as insurance!