hdiutil, passwords and putty

I'm trying to use putty/plink to login to my mac (from a w7 pc) and use hdiutil to mount an encrypted dmg. I've tried several ways:

plink -ssh user@server hdiutil attach -stdinpass file.dmg

I get prompted for the login password but when I enter it and press return there is no response. No further prompt for the dmg password.

putty -ssh user@server -m textfile

(where text file contains either the hdiutil command above, or names a shell script on the mac containing the command)

In this case I get prompted for the login password and it's accepted, but then the cmd window doesn't accept any further input.

I know that the login password is being accepted because I can echo some text before hdiutil command in the script.

Oddly, it's happy to DETACH a mounted dmg.

plink -ssh user@server hdiutil detach /Volumes/mounteddmg

takes the login password and ejects the volume on the mac. Works every time.

I keep thinking that the problem lies in hdiutil's -stdinpass argument, perhaps it's not seeing the incoming putty/plink connection as stdin?