Seems that the following tag (kudos @unor) will create a right-justified RTL paragraphs in a Tumblr post:
<div style="text-align: right;" dir="rtl">
See example.
Note:
In editing mode
, these paragraphs will still show left-justified, but the displayed post
will be right-justified.
Another Note:
This may seem obvious, but is worth mentioning, that the above tags should be inserted while editing the post in HTML view.