Hi JC,
The context node that you are using is a mixed node, so from this node you need to get value node so that you can cast the value into cl_bsp_wd_value_node.
Here's the sample code for getter and setter method of that attribute:
DATA:
lr_mixed TYPE REF TO cl_bsp_wd_mixed_node,
lr_value_node TYPE REF TO cl_bsp_wd_value_node.
lr_mixed ?= current or lr_mixed ?= iterator. " use either of the code i dont have system right now
lr_value_node ?= lr_mixed_node->if_bsp_wd_ext_property_access~get_value_node( ).
Here, from lr_value_node get the property access method and set the attribute value to VALUE parameter.
Thats it 
Thanks,
Bhushan