Is it safe to overfill temporary (6 weeks before OP) Mentor's TE with 40-50% of its maximal capacity without causing rupture?

Thank you for your question. You have a legitimate concern, and generally it is important to stick to the manufacturer recommendations. Although, in this instance, overfilling is ok. During my training, I wanted to see how much a mentor expander could be overfilled before it burst. I reached nearly 3 times the recommended fill volume and it still hadn't ruptured, but I physically could not inject any more fluid. The expanders are tough and that is why they are temporary, they aren't soft enough to be left in permanently. I hope this helps.- Dr. Bryson Richards

Although not recommended, significant overfilling of a breast implant with saline probably does not pose any significant risk for failure of the implant.

Overfilling tissue expanders is not dangerous and should not cause you any problems.  The goal is to stretch out your skin as much as possible.  Especially since this is a short term thing, you should not be worried.