That's not possible. sudo gainroot will open a new shell and then your alias is lost. Also on a security point of view, the user cannot interact with a root process. Which is good..
Not possible also. Leafpad uses a widget which takes Xwindow input. If it would take command line input, then we could do something like
popen("leafpad") and then use the descriptor to write into it.

In this case, we have to create use input at X level. The only way that I know (I use it at work) is xreplay or xnee.
xreplay < "go_to_leafpad_window,click file, click open, type $filename, click ok"

Ugly but saved me for GUI locked app.

edit: I think the leafpad that I updated for OS2008 takes the filename as parameter, if this is specific to leafpad.