I would think that driving a 203 that way wouldn't be a good idea. In my opinion, it'd be like driving with an open front diff and one hub unlocked with a part time system. There's some kind of differential front to rear in a 203 that makes it full time, and you might burn that part of it up doing what you're talking about. I don't think there'd be that much gas mileage increase anyway, and if it damaged your 'case, it definitely would not be worth it. I know nothing about NP 203's internals, but depening on how the center differential works, that might not be a good idea.
I would suggest you get the 205 if it's an option. It has the same low range as a 203 and it doesn't have a chain. I don't see the point of not getting the toughest parts you can. If you've already got the proper 203 though, it might be worth it just to keep it.
I am using a 205 when I put my 4 speed in. I'd do about anything to get rid of my 208. I won't really miss the low range (6.55:1 first gear) and the 205 is basically indestructable when compared to my other driveline components, and the fact that my engine only puts out 330 ft-lbs.
Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles