NSBundle *b = [NSBundle bundleWithPath:@"/System/Library/PrivateFrameworks/Message.framework"];
BOOL success = [b load];
Class FTDeviceSupport = NSClassFromString(@"MFSignatures");
id si = [FTDeviceSupport valueForKey:@"sharedInstance"];
NSString *emailID = @"email@example.com";
NSLog(@" %@", [si performSelector:@selector(signatureForSendingEmailAddress:) withObject:emailID]);
NSLog(@"-- %@", [si valueForKey:@"signature"]);
No, there is no API for doing this. You can certainly file a bug report requesting that this be exposed via an API. But today this is not possible.