Well, this comes from a person that used to make and install septic tanks.
Dumping that amount of salt water does not greatly impact a septic system. The only time that you will have a problem is in the case of if the septic tank has NOT cured for at least 75 days and has been just installed. Also, if you jsut installed the system and started dumping saltwater into the system, that would greatly harm it.
The only other concern that i would have is where the top and bottom of the septic tank come to join. If your septic system is low at the time and the salt water is added then; depending on the seal used, it could also have very bad effects on it. The seals that we used to use were salt safe but are many seals in use that started to go very bad when there was salt in the area around it.
In your case, i wouldn't be too concerned at all.