Comedy option:

    - (NSString *)format:(id)first, ... {
        NSUInteger argCount = 0;
        BOOL prevPercent = NO;
        for (NSUInteger i = 0; i < [self length]; ++i) {
            unichar c = [self characterAtIndex:i];
            if (prevPercent && c != '%')
                ++argCount;
            prevPercent = c == '%';
        }

        NSMutableArray *argArr = [NSMutableArray new];
        va_list args;
        va_start(args, first);
        for (NSUInteger i = 0; i < argCount; ++i)
            [argArr addObject:va_arg(args, id)];
        va_end(args);

        switch (argCount) {
            case 0: return [NSString stringWithFormat:self, first];
            case 1: return [NSString stringWithFormat:self, first, argArr[0]];
            case 2: return [NSString stringWithFormat:self, first, argArr[0], argArr[1]];
                // ...
        }
    }

Only supports NSObject arguments, obviously. If you actually wanted to do this approach you'd want to do a better job of parsing the format string and use a more cleverer way of storing the args.

Or, for the completely insane approach that probably doesn't actually work (on top of being entirely unportable):

    - (NSString *)format:(id)first, ... {
        NSUInteger argCount = ...;

        va_list args;
        va_start(args, first);
        void *argList = malloc(sizeof(id) * (argCount + 1));
        memcpy(argList, (void*)&first, sizeof(id));
        memcpy(argList + sizeof(id), &va_arg(args, void *), sizeof(id) * argCount);
        va_end(args);

        NSString *ret = [[NSString alloc] initWithFormat:self arguments:argList];
        free(argList);
        return ret;
    }