I've been skulking around the MMM site and forums for a long time, but your post on one of my "favorite" topics - US taxation of US expats - is bringing me out of the woodworks (though - spoiler alert - I don't have a silver bullet). I'm an American living and working in Switzerland and also receive employer contributions to my pension plan here. The tax advice I've received for my own situation is that the contributions need to be included in my US taxable income in the year of contribution (not when repatriated!) since the Swiss employer pension plan, unlike a 401k, is not a "qualified" retirement plan under US tax law. If not a "qualified" plan, then the special US tax treatment, i.e. "pre-tax" treatment of employer and employee contributions, does not apply (even if Swiss law allows pre-tax treatment in my Swiss tax return). The annual employer contributions are treated as normal income in my US return like my salary. For US tax purposes, the "qualified" vs. "non-qualified" characterization is very important whereas the treatment under Swiss (or French) tax law is irrelevant for the most part. Is there a specific reason why you said that the "qualified" vs. "non-qualified" terms don't apply to your case?
As a potential bright side, you mentioned that you currently don't pay any US tax, which leads me to believe that you pay so much tax in France that you're overflowing with Foreign Tax Credits (or under the FEIE threshold, though this sounds less likely if you care about your marginal tax rate). If this is the case, you may owe little or no tax even if you do ultimately have to declare the contributions as income. But more specifics about your tax situation are needed to determine this.
Unfortunately, the best advice I can give you is to check with a US CPA specialized in expat taxes. As is often the case with tax issues, the impact is heavily dependent on individual facts and circumstances, and there maybe more than one way to proceed.