The second is unusual, but possible, if the 'have been' refers - in effect - to something that's already happened (the arrangement to work late):
'My manager offered me the choice. I could have been working [that is, " I could have been put down on the roster to work..."] late tomorrow, but I chose to go home at the normal time and come in on Saturday instead.'
*PS See later discussion. Context can justify this.
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