Setting a variable directly somewhere inside the animation graph is impossible as far as I know. This is because the nodes you see on the graph are actually special classes, not functions on the animation instance class. If you want to access the transform of a bone at a specific point in the anim graph you need to create a custom anim graph node. The class you want to look at (or a struct actually) is FAnimNode_SkeletalControlBase. As an example you can look at the implementation of nodes such as FAnimNode_CopyBone or FAnimNode_ModifyBone. In there there are 3 main functions: InitializeBoneReferences, IsValidToEvaluate and EvaluateSkeletalControl_AnyThread. Inside the initialize function you want to set the bone references. In the copy bone implementation it looks like this:
// AnimNode_CopyBone.h
UPROPERTY(EditAnywhere, Category = Copy)
FBoneReference SourceBone;
UPROPERTY(EditAnywhere, Category=Copy)
FBoneReference TargetBone;
// AnimNode_CopyBone.cpp
SourceBone.Initialize(RequiredBones);
TargetBone.Initialize(RequiredBones);
In the IsValidToEvaluate function you check whether those references are valid. Again, here is the example from the copy bone:
// AnimNode_CopyBone.cpp
return (TargetBone.IsValidToEvaluate(RequiredBones) && (TargetBone==SourceBone || SourceBone.IsValidToEvaluate(RequiredBones)));
EvaluateSkeletalControl_AnyThread is the function where all the logic happens. Here is the simplified version of what you need to do to access the bone transforms:
void FAnimNode_CustomNode::EvaluateSkeletalControl_AnyThread(FComponentSpacePoseContext& Output, TArray<FBoneTransform>& OutBoneTransforms)
{
const FBoneContainer& BoneContainer = Output.Pose.GetPose().GetBoneContainer();
FCompactPoseBoneIndex BoneIndex = BoneRef.GetCompactPoseIndex(BoneContainer);
FTransform BoneTransform = Output.Pose.GetComponentSpaceTransform(BoneIndex);
where BoneRef is the FBoneReference variable defined in the header file.
To change the bone transform you do something like this:
OutBoneTransforms.Add(FBoneTransform(BoneIndex, SomeTransform));
where BoneIndex is the index of the bone you want to modify.
You can add more variables to the node, if you give them the EditAnywhere specifier they will be visible in blueprints. You can also add meta=(PinShownByDefault) specifier to make them be exposed as a pin for the node.
You can do whatever you want with that transform in this function but again, you cannot access that value anywhere outside this function. If you need to do so, you either have to refactor your code so the whole logic that requires this variable happens inside this node or, as a dirty workaround, you can add a bone to your skeleton and set its transform to the transform you calculated here.
HOWEVER, that is not the end. You have the node logic pretty much complete but to actually make it usable in the graph, you need to create a subclass of class UAnimGraphNode_SkeletalControlBase. The class for your node should look like this:
// .h
UCLASS(MinimalAPI)
class UAnimGraphNode_CustomNode : public UAnimGraphNode_SkeletalControlBase
{
GENERATED_UCLASS_BODY()
UPROPERTY(EditAnywhere, Category=Settings)
FAnimNode_YourNode Node;
virtual FText GetNodeTitle(ENodeTitleType::Type TitleType) const override;
virtual FText GetTooltipText() const override;
// .cpp
#define LOCTEXT_NAMESPACE "A3Nodes"
UAnimGraphNode_CustomNode::UAnimGraphNode_CustomNode(const FObjectInitializer& ObjectInitializer)
: Super(ObjectInitializer)
{
}
FText UAnimGraphNode_CustomNode::GetTooltipText() const
{
return LOCTEXT("AnimGraphNode_YourNode_Tooltip", "Tooltip");
}
FText UAnimGraphNode_YourNode::GetNodeTitle(ENodeTitleType::Type TitleType) const
{
return "YourNodeName";
}
#undef LOCTEXT_NAMESPACE
In the GetNodeTitle function you can put way more stuff to make your node look fancy, for an example you can check the AnimGraphNode_CopyBone.cpp file. After you do all that you need to add a few dependencies to your .build.cs file:
PublicDependecyModuleNames.AddRange(new string[] { "AnimGraphRuntime", "AnimGraph", "BlueprintGraph", "Kismet", "KismetCompiler", "UnrealEd" })
All this should allow you to add your node to the animation graph and use it. OP has probably already moved on with his life but I hope that others that just like me came across this post 7 years later will find it useful 
