You may not know that Marionnet already shares some files between host and guests for internal use, this is done through hostfs, a UML feature that allows to mount a host file system that you provided as a boot option. Fortunately for us Marionnet already does all the job and all we need to do is to use this shared directory, provided that you understand that this is a little hack and not a clean solution. Maybe something nicer and graphical will be provided in future.
The directory is
/tmp/marionnet-XXXXX/nome_of_project/hostfs/Y/ where X are some random number and Y too. Inside you will find 2 files, don't touch them!
For example I add to test a simple Bittorrent-like application:
host:~$ for i in /tmp/marionnet*/*/hostfs/*/ ; do cp myapp $i; done
guest:# cp /mnt/hostfs/myapp .
Easy, ain't it?