This is more of a discussion than a feature request.
Field level encryption exists in Accumulo , the HBase derivative that was created by the DoD for use within the US Government.
While I don't know the internal layout of the cell within MapRDB, (either table format), I would imagine that you could extend it to contain an additional byte to contain an encrypted copy of the key. So on encrypt, you use the key to encrypt the data as well as the key and store both. To decrypt, you decrypt the stored key and if it matches the key, then you can decrypt the cell's value and return it otherwise return NULL.
In addition to this, you will need to make the encryption call to allow plugins. This way you can choose the right encryption software. (e.g Triple DES or something stronger)
There's more, but this is just the start.