She grined at him, a shark's grin. "If I'd had an opportunity, I was going to make a play."
Would "I was going to make a play." mean "I would make a play" or "I would have made a play"?
To refer to unreal past, right? Is it colloquial?
"............,I was going to make a play" for "............,I would have made a play"?
I don't know what the context of the piece you were quoting was. In some sports, you can say that you made a play and it's a perfectly normal, acceptable phrase. You can "make a play for someone", meaning that you flirt with them, ask them out on a date or something similar. That would probably be described as colloquial but, as always, it depends on context.
I've not come across such a conditional sentence in grammar text books. Is it acceptable in writing?