Can a Hidden Guards! be played to summon a token elsewhere?

No, you cannot play a hidden Guards! to summon a token at a different battlefield.

According to Rule 737.1.d.3: "If a hidden spell or a play effect of a hidden permanent causes you to play a unit, you must choose to play that unit at that battlefield [where the card was hidden]."

Because Guards! is a spell with the Hidden keyword, when you play it from its facedown position, any units it creates (in this case, the 2 [S] Sand Soldier unit token) must be played at the same battlefield where the Guards! card was hidden.