Turns out you can add methods to the block classes

gistfile1.m

void foo(id self, SEL _cmd);
void foo(id self, SEL _cmd)
{
    NSLog(@"foo");
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    NSString * flubber = @"flubber";
    void (^barGlobal)(void) = ^{ NSLog(@"bar"); };
    void (^barStack)(void) = ^{ NSLog(@"bar %@", flubber); };
    
    class_addMethod([barGlobal class], @selector(foo), (IMP)foo, "v@:");
    class_addMethod([barStack class], @selector(foo), (IMP)foo, "v@:");
    class_addMethod([[barStack copy] class], @selector(foo), (IMP)foo, "v@:");
    
    barGlobal();
    [barGlobal foo];
    [[barGlobal copy] foo];
    
    barStack();
    [barStack foo];
    [[barStack copy] foo];
}