My 401k plan allows me to set up 'back-up after tax' (BUAT) contributions that go into a separate, after-tax bucket of my 401k. My understanding is that this is not a universal feature of 401k and would depend on your employer's specific plan. It was just introduced to our plan this year. I'm also not clear on what kind of restrictions there are on using the BUAT feature. You may run into trouble if you try to put a huge extra contribution to your 401k.
Based on the 'back up' in the name, I believe the feature is intended to help employees max out on their 401k contribution for the year while still not missing out on any of the employer match. That's how I am using it personally. I'm not sure exactly how my salary could change over the year, and what sort of bonus I will get. Also, the only lever I have to adjust my contributions is a single % of paycheck number. So it makes sense to set that % a little higher than what I think I'll need to max out the 401k (17.5k this year). Without the BUAT feature, I might miss out on the employer match, since they match my contribution each paycheck. I set my BUAT at the minimum % of paycheck necessary to get all the employer match so that after I max out the 401k, I don't lose out on the freebies. I think it would work to put my BUAT at a very high %, which is essentially what you're talking about doing, but I haven't asked since I'm not interested. I believe (but I'm not sure) those after-tax contributions are still subject to the early withdrawal penalties, so that's why I'm not anxious to over-contribute.
I know some other companies wait until the end of the year to see how much you contribute and THEN put in their matching amount, so in that case the BUAT is not necessary/not offered.