I thought about this before or something similar, I worked PRN at another company as well as a full time job. Anyways, I'm able to max 401k at full time job but also put money into PRN's 401k. This allowed me to put in over the IRA max since I contributed to both. I was told that I shouldn't do this since I got taxed for the extra amount that went over the limit then I got taxed again when I took the money out, so taxed twice. But I went ahead and did it. Anyways since I stopped the PRN job due to current job taking so much of my time, I just rolled PRN 401k into my roth.
A round about way of saying this, but isn't this thread just the same as what I did when I worked 2 jobs and put money into both 401ks? If so, the IRS didn't really care that I over-contributed just as long as I paid my taxes on the amount over their deferred limit.
edit:@wile's link, in my case above with over contribution, and paying the taxes for it, isn't this the same as that option 3 in his link? And if it works that way, I might just pick up seasonal jobs or other PRN jobs and then each time I'm done, I can "quit" and roll it over into a roth, pay the taxes on it, then apply for job again next season. It's fairly easy to justify the PRN job "quitting" too, at least for me. I helped out when someone is off for maternity/family leave which is normally a few months. After they come back, I'm "no longer needed" so I let the PRN spot go. This could be the same for temp agencies too?
edit: it might depend on how much you over contribute too? I just worked twice a month but even then I made about $8,000/year with about $5,000 into the 2nd 401k. Maybe it was so little that it didn't draw IRS's attention?