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It's not just about the weight, it's also about the distribution of the weight, right?
You're right the weight distribution or more importantly the distance between the mount and centre of mass plays a role, in fact a long enough 680g would rip the mount off a professional DSLR, but I don't think we're in that territory.
Picking my Canon 100L macro up, which is about the same weight and length as the Samyang 24 TS it just doesn't feel like it ought to be able to do damage to the mount. Especially not with a metal body A7r.
However given it's a TS lens you'd probably want to buy the Canon or Nikon version and put it on an adaptor with a tripod socket so you can shift the camera independent of the lens. This will also take any strain off the mount (and any adaptor slop you can compensate for with the tilt mechanism, so it's a win all round )