With MultiBit HD we have added two options to Sign and Verify that do what you want:
On the sign message, you've got a 'Copy All' which copies a well formed signed bitcoin message to your paster buffer e.g.
-----BEGIN BITCOIN SIGNED MESSAGE-----
Hello world
-----BEGIN BITCOIN SIGNATURE-----
Version: MultiBit HD (0.0.8beta)
Comment:
https://multibit.orgAddress: 1JY5Bp3RFLwGDp4Za4uwgQgcnP7JPBYKuS
IHZi6DhViCmJ4xPkVn7fNhDh1FpAs2Wg2lPdmRNneMp4e7FADAzWdvS/jV4ssH8g9lwTl+DkryYWMFAkvk9XpmI=
-----END BITCOIN SIGNATURE-----
Then on 'Verify message' you have a 'Paste all' that does the reverse i.e. it parses a signed bitcoin message and populates the fields, ready for you to Verify.