I wonder if they'll finish the support for basic data import commands with Mongo. Currently on Duostack I have yet to find a clear way to load dev data via all the db.copyDatabase or mongoimport/mongostore commands.
You can run mongoimport locally and specify your DotCloud MongoDB service as the remote host. You could also SSH to your MongoDB service (run "dotcloud ssh <namespace.deployment>") and do the same.
One thing you may want to consider adding to your client is a "mongotunnel" command which will set up an ssh tunnel with compression between the user's computer and your mongodb servers. I've noticed some significant performance benefit to this vs. just accessing mongod normally over the internet, mostly due to the abysmal upload rate on my cable modem. Also this allows you to avoid sending data in plain-text over the open internet.