I've got an idea, and I just put a script together to provide a possible solution.
I posted it at my website: RUNAS Example
Click the link on the main page called RUNAS.

It seems to work well. It uses the SENDKEYS function.

I'll also post the script here for your reading pleasure.

code:
;RUNAS USER
;Kixtart script put together by Jason L Stenklyft
;Jason@Stenklyft.com
;example of using sendkeys to pass a password to runas
;not recommended for production networks, due to security risks

;Requirements to test this script
;Create a make a user on the local box named LUser and set the password to password

SETTITLE ("SENDKEYS ROCKS!")
  ;set self window title
RUN '%COMSPEC% /e:1024 /c runas /user:@WKSTA\LUser "notepad"'
  ;launch some notepad action as LUser
$trash= SetFocus ("SENDKEYS ROCKS!")
  ;set focus back to self
$trash= SendKeys("password{ENTER}")
  ;send the password and enter
sleep 3
;wait a few for notepad to launch
If SetFocus ("Untitled - Notepad") = 0
;if we can now setfocus to notepad, write some text
 $trash = SendKeys("Would you like to play a game?")
Endif
_________________________
Jason Lee Stenklyft Jason@Stenklyft.com Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones. The Stenklyft.com domain and all associated e-mail addresses are located in the State of Washington, and sending mail to addresses at this domain is subject to the provisions of the Revised Code of Washington.