Another name that occurred to me -- and is probably my favorite so far -- is ontofile, which is still simple enough to not need hyphens, communicates intent (appending something onto a file), and worms the word to in there, painting it with the to:output / from:input correlation. Thoughts?
+1! ontofile is a great name, in my opinion, for all the reasons you listed.
I searched for a good portmanteau in the vein of mappend, but I don't think there is one. fappend? Sounds like frappuchino. filepend is decent, but I think I prefer ontofile.
