Got an EX raid invitation for an inaccessible location. Is there anything I can do?

I received an EX raid pass:

However, the park will not open on October 1st, thereby making it impossible to participate. What can I do? Can the date be changed? Any other suggestions?

Since the gym is inaccessible during the time of the EX-Raid, your best bet is to submit a ticket with Niantic and explain the situation. According to the users on reddit, Niantic will cancel the raid and compensate you.

As evidence you will be compensated, a trainer I know received a few items after an EX-Raid they were invited to was canceled for unknown reasons just before the event.

All you can do is try to contact Niantic asking for an alternative raid or time, but, since the question is quite time sensitive, I doubt they will respond to your needs in time. I got a reply within two weeks when I contacted them on another issue.

Like in normal raids, you should not bother trying to get to a raid where the attendance of other players isn't expected to be high enough. After all you can't beat it alone.