EDIT: This no longer works. As foreseen in the initial answer, the ABI appears to have changed out from under this answer
Played around for a bit and got it to work — Double checked for leaks or abandoned memory and didn’t see any.
NSArray *fixedArguments = [[NSArray alloc] initWithObjects: @"foo", @"bar", @"baz", nil];
NSRange range = NSMakeRange(0, [fixedArguments count]);
NSMutableData* data = [NSMutableData dataWithLength: sizeof(id) * [fixedArguments count]];
[fixedArguments getObjects: (__unsafe_unretained id *)data.mutableBytes range:range];
NSString* content = [[NSString alloc] initWithFormat: @"1: %@ 2: %@ 3: %@" arguments: data.mutableBytes];
NSLog(@"%@", content);
I like to (ab)use NSMutableData like this to get retain/release semantics on an arbitrary chunk of memory — It’s not necessarily relevant to the issue at hand, but it’s a neat little trick.
As a note to future readers: Faking up a va_list like this happens to work with the current ABI for MacOS and iOS, but in general it’s not portable, and not a good approach.