Can a hidden unit (like Teemo) be played from a battlefield when Mageseeker Warden is preventing units from being played to battlefields?

Ruling: A hidden unit cannot be played at all when Mageseeker Warden is preventing units from being played to that battlefield. The hidden unit remains hidden and cannot be revealed/played.

Nuances:

• Initially there was debate about whether the "if able" language in the FAQ allowed the unit to default to base, but this was clarified in updated rules
• The updated comprehensive rules state "A hidden unit must be played to that battlefield" without the "if able" qualifier
• Hiding a card is not the same as playing a card, so you cannot hide cards at a battlefield where Warden is present in the first place
• If you lose control of the battlefield, the hidden card gets trashed like any other unrevealed hidden card