Assign To MVG has to show dynamic names in the MVG applet

New requirement --New Lead create .user fills required data like type, sub type and other details . Default the person who creates the lead gonna be the primary in the assign to field . user change the assign to someother user depending on some condition [Default on left panel he can see all the user in the mvg]

Req:

eg: Lead of type 1 has to assign to user 1 and lead of type 2 has to assign to user 2 .

Current situation :sadmin creates a new lead and fill all the corresponding fields and type selected as "Type 1" . Sadmin has to assign to user 1 . sadmin click on assign to field and sadmin can able to view user 1 and user 2 in the mvg applet .

But as per our requirement i have to show only user 1 in the MVG . I dont want to show user 2 for lead type of TYPE 1 . User 2 only has to show for Lead type of type 2 .

How i can achieve this . ???

thanks and regards

Joseph thomas