Both of these options have their own drawbacks, mainly cost. I just started using a couple 6 gallon better bottles because I'd like to be able to see what is going on in there. I'm not about to abandon them or use both on a single batch. I'd like to know what you think of my idea now that you know I'm not talking about puring sanitizer into my primary. The way I see it is you are essentially increasing the volume of your fermenter.
If you don't want to spend the money on a larger fermenter (an ale pail is about $15 or so, but a bigger glass carboy is $$$$), that does narrow your options. I LOVE my ale pail, and often don't even use my Better Bottles (except for 6 gallon batches of wine in secondary), but I understand that once you buy them, you'd like use them.
In order of preference, I'd do:
1. Bigger fermenter
2. Accept that sometimes the beer will blow off in a 6 gallon carboy- keep the temperature at the low end of the yeast's optimum temperature and the blow off should be minimized. Discard the blow off.
3. Slightly smaller batches- make 4.75 gallon batches. Scale the recipe to that size, and not have any blow off.
4. Use the BB for most of the batch, use a 1 gallon jug (those 4L Carlo Rossi jugs work fine with a #6 stopper) for the extra wort.
I don't think I'd mess around with an airlock on a blow off container, etc. to save a little wort.
Maybe it would work, but it means making sure that the blow off tube is completely sanitized (krausen really gunks those up), the blow off container is sanitized, that the weather favors it (as to not get a vacuum between the blow off container and fermenter during a cold front, etc), and then if you actually get beer from the blow off, you don't want to pour it (because of aeration/oxidation issues) so you'd have to rack it. And the krausen that fell back into the beer would be much more predominant in the vessel that held the blow off material. In short, I don't see any advantage, but I see plenty of disadvantages.