Also, just adding to this, since I did have a look and indeed Effect Target is not something you can expose without stealing the node C++ and re-compiling it with the exposed pin.
You can always just use a blend by BoneControlSpace
you can get a switch from typing switch bone control space. and promote the selection pin to a var to get the drop down variable defined.
you can then move the pin to blend poses by Bone Control space. Right click and add pins to expose the various types.
Feed differently configured IK nodes into the different poses.
Pin the variable you created.
Set the default pose to the baseline animation.
And you worked around it in BP without having to compile a specific BP class with the exposed node.
Keep in mind that you are probably killing fast path by doing this.If the thunderbolt sign is gone from the blend you may be better off exposing the pin in C++