So how do I go about getting my bosses to implement this, as I will be getting a bonus next month and will already have maxed my IRA and 401k?
You sound like the perfect candidate then. Go to your smartest/most logical boss tomorrow. Lead off the conversation with something like: "I think I have a way to save the company, you, and I some taxes." Tell him you believe your safe harbor contribution of 3% opens up a gateway for elective contributions in excess of 3% (mine allows for up to 9% total) and you would like to discuss with him and your 401k administrators the possibility of you getting this excess contribution as part of your total compensation package this year. (Aka, in exchange for part/all ofyour bonus, but don't officially document it as such, and don't tell this to the 401k administrator because it irrelevant, all they need to know is boss 1&2, and frugalnacho all get an extra 6%).
Younger people, including me, have another hurdle. These contributions are an allowable, elective disriminatory contribution. This means you are allowed to discriminate but only a little. They test this excess benefit for the value of the benefit when you reach 59.5. So if you are 55 and you get $5k, no sweat, but if you are 30 and get $5k that's worth like $50 or something. The 401k administrator knows all the rules and will tell you what you can and can't do.
Good luck. If it works you could be a hero to the bosses. Or maybe they already do it and don't want you to know about it. But it will save them payroll tax so they should be all for it.