Unfortunately, it's not possible.
See this question: Is it possible to add an HTML link in the body of a MAILTO link
The answer there refers to RFC 2368, whose second section reads, in part:
The special hname "body" indicates that the associated hvalue is the
body of the message. The "body" hname should contain the content for
the first text/plain body part of the message. The mailto URL is
primarily intended for generation of short text messages that are
actually the content of automatic processing (such as "subscribe"
messages for mailing lists), not general MIME bodies.
So, according to the standard, the body has to be in plain text. That means that you're at the mercy of the client software: if it conforms to the standard, it will treat your body text like plain text, and behave in the default way.
I tried a lot of things on my local system (Outlook 2003), including changing my Normal.dot to be RTL, and configuring Outlook to use MS Word for new messages, and changing Outlook options to do everything RTL, but nothing worked, even as a workaround.