picture p;
p := textext("MetaPost is fun!") shifted (10cm,10cm);

My current understanding of the required steps :


1- fill p with the shadow color

2- write it to an external metapost file (with savebuffer ?) ready for png export (outputformat := "png »;)

3- with lua, os.execute, and imagemagick prepare the shadow
  - extent the png file with a transparent background, to have room for blurring 
  - blur it

4- import this png in context / Metapost (externalfigure), and center it with p figure

5- shift it according to the desired shadows distance and angle

6- apply the initial bounding box of p to the shadow, draw the shadow, draw the p picture 


Actually, all this seems long and tedious, and contrasts with the already existing links between MetaPost / MetaFun and cairo + libpng.