Unfortunately MultiLeaders do not have an obliqueangle property. This is a property of the MultiLeader Textstyle. They also don't have a blockscale property, at least on my version (2012)
Your two options are make a text style with the required obliquing angle and change the text style or; as the text is MText, make an MText object and give it the required obliquing angle. By extracting the text string from Mtext object you will find what the required in string format code is. If you forget the curly braces then it's should be a simple matter of
Code:
(vlax-put-property obj 'textstring (strcat "{" "format code" (vlax-get-property obj 'textstring) "}"))
Where "format code" is of the form "\\???;"
If you want to find out properties and applicable methods of objects use
Code:
(vlax-dump-object (vlax-ename->vla-object (car (entsel "\nSelect Object to Dump : "))) T)