How Do i implement an alternative for the Privatemsg Bulkmail module?
Looks like there are currently no plans for a D7 version of the Privatemsg Bulkmail module you are referring to. However, I'd use the integration of the Privatemsg module with Rules instead (make sure to checkout this community docu, and its child pages ...), together with the VBO module.

For more details, refer to the video about Using Rules components with VBO. Possibly you also want to look at the 2 prior videos and/or the 2 subsequent videos (all about Views Bulk Operations), i.e.:


Introducing Views Bulk Operations.
More details about VBO settings.
Using Rules componens with more than one parameter.
Using VBO to load list of objects into Rules.


In this specific case;


use Views bulk operations to create the list of your users to be targeted.
use Rules action "Send a message" (provided by Privatemsg module).


PS: yet another great usage of the Rules module, in this case in combination with the Privatemsg module. So I wouldn't worry too much about the Privatemsg Bulkmail module not being available for D7.