The phrases are not equivalent.
Could be/could have been and mould be/might have been may both be used as you intend, to introduce a simile, more or less equivalent to "It was as if he were/had been".
Might as well been is ungrammatical: might requires an infinitival complement. Present-tense might as well VERB and past-tense might as well have VERBen are always used to express a comparison to something relatively undesirable or ineffective, "Things would be no better if SUBJECT VERBed/had VERBen":
I might as well go home, I'm not accomplishing anything here.
He's not going to listen, she might as well save her breath.
I might as well be dead for all the attention he pays to my ideas.