Can you escape a grapple during a time stop (without teleporting or similar effects)?

Some context: during our last game a hidden creature charged and grappled (with improved grab) one of the PCs (a 20th-level Sha-ir/Cleric/Dweomerkeeper) during the surprise round. After that this same PC won initiative and the first thing he did was casting time stop. We decided to stop the game there for several reasons, it was already time, one other player had already left and this was an important encounter he didn’t want to miss, and I wasn’t really sure how to rule this situation yet.

So regarding the question itself, say you’re in a grapple with another creature, and during your turn you cast time stop (since it only has verbal components you can cast it while grappling with a DC 29 Concentration check), but you’re inside the area of a forbiddance spell, so you can’t just teleport out.

Could you escape the grapple while time is stopped using the normal method (by making a grapple or escapism check)? I’m guessing that since the opponent can’t move or be moved he can’t make a grapple check either so you can’t even try the opposed check.

Would casting freedom of movement be of any help at all? You automatically succeed on the grapple check to free yourself, but I don’t think you can make that check to begin with.

What other methods could you use to escape before the time stop ends?