Can hidden units be played to Rockfall Path?

Yes, it is legal to hide a unit card facedown on Rockfall Path. However, you cannot play that unit while it is hidden there.

According to the RiftJudge FAQ (ID 6500):

• Hiding a unit card facedown on Rockfall Path is legal.
• When you attempt to play that unit, it is an illegal activation.
• The play is rewound, the card must remain hidden and cannot be played, and it gets trashed when you lose control of the battlefield.

If a spell or ability instructs you to play a unit to Rockfall Path, you can shortcut the instruction to simply not execute (the spell/ability resolves to no effect).