I've got a FM layout with a portal I want to show related rows from a MySQL table.

It works fine as long as the relation is based off a "single" entry.

FM field has "Open" in it

MySQL field has "Open" in it

My problem is I'd like to expand this to show fields with one of two values, such as "Open" or "Pending"

Only way I can think is with another FM table with a calculation in it, but that calc can't be used in a relationship (non-indexable)

The parent side of a relationship can have an unindexed field.

But if you want an "or" relationship, you need a multi-key in the parent side. In your case, a FM field that has:

Open

Pending

in the field (carriage returns). That'll match all records in the child with either Open or Pending in the match field.