I have used a 3rd party battery in an A700 with no problems. If you use a vertical grip you will have problems. See this Thread
which explains the issues.
As Fixxxer says, why take the chance of voiding the warranty on such an expensive camera. I'm not sure if doing so will void the warranty, but it may, just as cleaning your sensor with anything but a blower may void your warranty.
Sony A7, A850, A77, A700 || Sony>> 70-300G, 28-75 f2.8 SAM, 16-80CZ, 50 f1.4, FE 28-70 OSS, FE 24-70CZ f4 OSS, LA-EA4 || Minolta>> 300 f4 G HS, 200 f2.8 G HS, 100 f2.8 (D) Macro, 50 f1.7, 28 f2.8, 28-135 f4-4.5, 70-210 f4, 500 f8 Reflex, TC x 1.4 HS, TC x 2 HS. || Sigma 21-35 f3.5-4.2. || Tamron SP 24-135.