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Thanks for asking.
I googled around a bit on this topic and found this:
http://macscripter.net/viewtopic.php?id=24745
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The StandardAdditions OSAX -- part of the standard Mac OS installation -- includes a suite of six commands called File Read/Write.
[..]
open for access: open an access to a file for the other File Read/Write commands below."
-- quote --
In a nutshell:
"open for access" is not part of AppleScript itself, but instead is part of an OSAX scripting addition.
Currently, Remote Buddy doesn't seem to load OSAX scripting additions, so that these commands are not available for use from inside Remote Buddy. What you could do, of course, is to use AppleScript Editor to create executable code from your AppleScript and run that as part of your custom action.
Best regards,
Felix Schwarz