Always best to go to the source. The explanation of the proposed regs say (verbatim) in the Proposed Effective/Applicability Date" section:
These regulations are proposed to apply to taxable years beginning on or after the date of publication of the Treasury decision adopting these rules as final regulations in the Federal Register. Taxpayers, however, may rely on these proposed regulations for periods preceding the proposed applicability date. If, and to the extent, the final regulations are more restrictive than the rules in these proposed regulations, those provisions of the final regulations will be applied without retroactive effect.
First, I read the first part of that to say that WHEN FINAL, the regulations will apply only to taxable years beginning after publication. So, if they are finalized in 2017, that would be 1/1/18. Simple enough.
The part after the "however" says we can rely on these "proposed regulations" (without subsequent consequences if the final regs differ from the proposed) for "periods preceding" (emphasis added) the proposed effective date.
Now, words have meanings and "legal" interpretation says you use the ordinary meaning of words when interpreting them (unless the context requires otherwise). The proposal talks about "periods" preceding the effective date (i.e. it uses the plural) and it does not restrict those "periods" to only those occurring coincident with (2017) or after publication of the proposed regs. In other words, literally read, it could apply to any periods all the way back to the big bang.
I'm not advocating any position here - we are still evaluating how this applies (or how we want it to apply) to our book of business, but an argument certainly can be made that you can rely on the proposed regulations for periods in the past - such as a 2016 contribution that has not yet been funded (to the extent there isn't some other prohibition against the underlying act of funding a contribution to the past period).
I consider myself a "recovering" ERISA attorney (I work in-house with a service provider and not as part of the corporate legal department).