I agree with Kevin, there is no way you can set foot in the USA without financial (oops, I meant health) insurance.
Unfortunately, the way that I understand that PPACA is structured, it is all or nothing regarding residency. Which means a part-time USA resident must "anchor" their health insurance coverage in the USA and not abroad. A policy written by a Mexican insurer will definitely not qualify. In fact, I don't even think real First World international policies like BUPA will qualify. But this is only informed speculation on my part, I don't think anyone really knows for sure at this point.
Also, some short term medical policies, like that offered by Assurant Health (one of the options in my list that has been available up until now), may no longer exist circa 2014 (again, I am not sure, but at the very least the market for these policies will be drastically reduced and these policies cannot qualify for PPACA certification). In that case, you would only be left with options like World Nomads that Kevin mentioned (and they have a good reputation among Mexican expats). So the health insurance options for true expats who want temporary USA coverage will possibly be reduced.
Regarding residency, I would take a close look at the Bona-Fide Residence test which would allow you to spend more than 30 days per year in the USA. But it is more for long term expats, you don't even qualify until you have spent a full tax year residing abroad (you can amend back taxes, however):
Foreign Earned Income Exclusion - Bona Fide Residence Test
I would welcome other informed opinions on what I have stated here. I am not an expert.