Wouldn't the "de facto standard" actually be PDF? I know since you are defining terms yourself to your own ends that you are skewing the meaning to relate specifically to mobile devices but the truth is that PDF is the most widely used format out there combined with the most platforms.
Even if Mobipocket is the "de facto standard", there is little reason to argue to support it as an archival standard. This reminds me of a recent conversation on VHS/Betamax, and quite frankly the argument for Mobipocket would be like those that argued for DIVX over DVD when it came out, and arguing for VHS would be akin to arguing for PDF...
I own a Sony and would NEVER argue for BBeB/LRF to be an archival standard. An archival standard needs to be open and (to reiterate Zelda) non-proprietary in order to be somewhat futureproof.
As for current buy-once formats, if epub is unavailable I would think LIT would be the best option.
-MJ