I'm here to convince you to stop looking for an answer. Really very few people sets their password using Arabic character (sometimes even the UserName). This is because most password systems are very bad in handling non-latin characters. These system usually assumes passwords are in ASCII not Unicode especially old DB systems. Moreover, keep in mind that sometimes the user finds some difficulty to switch the keyboard to another language layout. This problem used to happen in Windows Login long ago.
These historical and chronic compatibility issues discouraged most people from using non-latin characters in their passwords. I've lived enough and dealt with lots different people (really a lot of people... people with zero computer skills, people using devices in Arabic interface, people with zero English... etc), I have still never heard someone sets his password in Arabic characters... not in the late 90's not even this era. Since the majority sets their password in latin characters, I believe that all login interfaces should be 'layouted' in LTR and also left aligned.
Another thing password field converts text to stars, so there is no difference between starts from right-to-left and starts from left-to-right (sadly mark down down't support align, so please click on run):
<p align="right">******</p>
<p align="left">******</p>
More technical info here