You can actually do this now.
Step 1: create a directory, say $HOME/aglobal
Step 2: copy any file(s) you want to be always present into that directory, for example, MNI152_2009_template.nii.gz
Step 3: in your $HOME/.afnirc file, add the line (to the ***ENVIRONMENT section)
AFNI_GLOBAL_SESSION = /Users/rwcox/aglobal
When you start AFNI, the files in this global session will always appear in the dataset choosers at the end. I personally use this "trick" to put the MNI nonlinear template (mentioned in Step 2) as a universal underlay.
