I don't think you can send messages outside the house, but I'm sure the contestants can convey their concerns to BB in the confession room. They can always speak to the camera or into their mics when they have an urgent issue that needs to discussed.
The matter of rashmi's keys was ofcourse quite sensitive, so she spoke about it in the camera. BB might have conveyed the message to her of it being replaced and handed to the production team.
Regarding asim, if he was genuinely worried about his brother's whereabouts he might have discussed that. It could have been passed forward.
Now, paras sending a letter is a loophole which I hope to figure out in the WKV. If salman accuses paras of sending the letter to his gf, then he will also have to accept the incompetence of the bb team that allowed it to happen!