From what I've found you can't reference properties from a custom command.
you could create a generic tail command that works on files and uses some of the other '%' operators like %p, etc...
The docs tell me about %h, which I understand, and %f and %p which I am unclear on. Is there a more thorough reference somewhere?
%h The names of selected servers on which the command should run.
%p The full path to the selected files or directories on which the command should run. This path does not include the server name. For example, a path might read /c/winnt rather than //host1/c/winnt.
%f Selected files or directories on which the command should run, excluding the path to those files or directories. For example, winnt rather than /c/winnt.
so %p is the path to the file or directory w/o the server... %f is basically just the file or directory name... in your case you want to make something like:
tail -f //%h%p
I'm not sure what %f would be used for - maybe if i needed to pass a file name to a command like 'ps -ef | grep %f' or something.