The rules state that if you are buried, you can take no other action until you dig yourself out. The jetpack card says "play at any time". Does playing a jetpack count as an action for the purposes of following the rules (albeit a free one), or do "play at any time" cards circumvent all action restrictions?
A rules faq on boardgamegeek here says you can.
Yes. While a buried player cannot take any action other than removing sand, using equipment doesn't take an action.
Since the card explicitly allows being played at any time, and cards trump general rules, I would say yes, you can play it when buried.
Thematically, buried obviously doesn't mean your entire body is covered in sand, it just means you can't move effectively due to the amount of sand. A jetpack cancels out needing to worry about the amount of sand. =)