Beche,
The primary thing to consider is the difference between what you can plan for and control, and what you can't plan for.
Explain to me why doing a deco dive with a single plus a pony bottle is unsafe. We have an entirely redundant gas source. Provided the usual gas rules are followed, how is that different from using twins?
To emphasize certain things that you already know (?), in your gas planning you will initially calculate rock bottom, and subtract from your total gas supply that gas quantity required to bring both you and your buddy to the surface from your deepest depth (should your buddy need your gas at that point). I'm going to set aside your deco gasses at this point, and assume that you would have these calculated properly and in separate bottles. As UP indicated, if only using one deco gas, you could always sling another 80 for that.
Most divers who plan overhead dives plan thirds. Overhead can be tangible (cave/wreck) or intangible (deco obligation). The fact that you plan to incur a deco obligation suggests thirds.
Therefore of the remaining gas, you'll plan thirds. You're planning decompression dives on wrecks, so even if you are not penetrating them, thirds is wise considering your remote location and your deco obligation, etc.
Thus the answer to your question, 'why is a deco dive with a pony unsafe?' First, 'pony' is imprecise, you have a decompression bottle. It doesn't factor into your backgas planning. A single 80 simply doesn't hold enough gas, after subtracting rock bottom and 2/3's of whats left after that, to allow you to safely perform deep dives - especially ones where you plan to incur a decompression obligation.
To come around full circle, the purpose of that gas planning and equipment is to attempt to be able to respond to things that you already realize could go wrong down there. The problem is that there are always things that can go wrong down there that you DON'T plan for, or cannot anticipate. To fail to plan for the foreseeable leaves you that much more at risk from those things that are not foreseeable.
As noted above, you have better options if you're going to do this sort of dive trip.
Here is a source for "travel bands" that will allow you to use two rental aluminum 80s supplied by your host with your own bp/wings:
http://www.aquaexplorers.com/double_bands.htm
Also, you can take (2) aluminum 40s. Arrange it beforehand with the airlines, and pack them in checked luggage with the valves removed.
Another thing, I've been to the western Carolines and up through the northern Micronesian islands. There are some truly wicked currents up there. If I were going back I'd carry some sort of surface marker bouy that I know would be seen for quite a ways. Something extremely large and extremely orange. You can find yourself nearly out of sight of the boat before you ever realize you're in trouble...some of those currents come up rather suddenly and they are truly impressive. Something to think about.
If you're going to do it, (to avoid contentious and copyrighted phrases) Do It Wisely. Or don't do it. But don't try to plan dives like those you've presented here without the right stuff.
For what its worth.
Doc
