Thank you for your question.
Our RadTextBox and RadPasswordBox components are intended to bring additional functionality to the native TextBox and PasswordBox - like validation, buttons, etc. The IsReadOnly property comes from the native TextBox and is not present in the native PasswordBox. I assume that the idea behind it is that when the TextBox is read only it will simply display a text that shouldn't be changed but should only be read and in the PasswordBox, simply displaying the dots wouldn't be such useful as in the TextBox.
What the password box does is, mask the input. If I understand your scenario right and since you don't want to use the IsEnabled properties in these controls, when you need the automatic login, you don't really have an input and therefore you can go without the password box. You can simply use the text box and fill it as many Bullet characters ((char)9679) as the length of the password is.
I hope this information help. Let me know if this works for you and if you need additional assistance.
the Telerik team