The following is my understanding of the situation. I know plenty of people who do the following, and am considering doing it myself, but would love to be corrected if and where I'm wrong:
If you forego the FEIE and instead declare your foreign income + FTC (assuming your country has a bilateral agreement with the US), you are eligible to contribute as far as the numbers are concerned. Actually contributing is a don't ask, don't tell situation (as for the rules against transacting across borders, the onus is on financial institutions and not individuals). You can contribute if you have an existing account that is linked to the US address you are using. If you mention that you're in a foreign country, they will refuse to work with you (this part I have experienced). If you don't mention it and they don't ask, the transactions go through (this is what I've heard). Presumably everything's hunky-dory when you return to the States in a matter of years, and your tax returns show that you had income from which to contribute.