Dynamic submission action attribute?

Is it possible to have the action attribute of the submission element be 
calculated or dynamic in some way? The scenario is that I would like to use 
an HTTP Put to submit the instance data and in some cases this will result 
in the creation of a new resource and in others it will be an update of an 
existing resource. Either way the action URI needs to be dynamically 
constructed (i.e. not hard-coded), but reading the spec it looks as though 
this is not possible, which would appear to limit the usefulness of the Put 
submit option.
Have I missed something?

Robert

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