I think that Gabriel Landini has a plugin that may be what you are looking
for:
See
http://www.dentistry.bham.ac.uk/landinig/software/software.htmlConvexHull1.txt: a morphological convex hull -- square shape (not the true
convex hull)
ConvexHull2.txt: a morphological convex hull -- diamond shape (not the true
convex hull)
Wish it helps.
JM