Given Dropbox's absolutely glacial pace of feature progression, no it would not be easy. I mean, all of dropbox runs off of one set of private keys...
Edit to add link to EFF ratings: https://www.eff.org/who-has-your-back-government-data-reques...
tarsnap is nerdish, secure and reasonably priced for what it provides IIRC. Haven't used it though but expect someone would have yelled out here if it was bad. (In fact the only one I've seen bashing it was patio11, -because it was too cheap and too nerdish.)
Dropbox was listed on PRISM documents. Are you kidding me?