additional formats or exec feature when encoding from pipe

[lvecsey@…]

Attempting to run mencoder with the pipe encoding example the program says it
expects data in the form of AVI, ASF or MPEG. Can .png data be easily inserted
into avi or asf? My preference here would be to have an -exec feature for
mencoder which would run a program with a frame number passed to it, and that
program need only return a .png or similar still image on standard out.