As you say, unreal ideas after "as if" require backshifting. Backshifting means that present tenses are changed to past, and present perfect is changed to past perfect. What happens when the main verb is already past?
When the main verb is past, and the idea is "unreal," you can't backshift any further than the past tense or the past perfect. This is true for the verb BE, the auxiliaries, and the modals. Here are some examples of "as if" sentences with an "unreal" idea in both present and past contexts:
1. She looks as if she were about to explode/She looked as if she were about to explode
2. He's dressed as if he were the gardener, not the homeowner/He was dressed as if he were the gardener, not the homeowner
3. He talks as if he didn't have a dime to his name/He talked as if he didn't have a dime to his name
4. He struts around as if he owned the place/He strutted around as if he owned the place
5. He looks as if he had lost his last friend/He looked as if he had lost his last friend
6. They sound as if they could make it all go away/They sounded as if they could make it all go away
In other words, the backshifted verb forms are the same for present and past unreal ideas.