I use an inline style with a smaller font and adjust the line-height and vertical-align. This example is what I use with a 16px body font, numbers would need adjusting for other font sizes. But it works in Outlook and every other client test we've done without needing MSO conditions, etc. It also doesn't create a weird line height gap if the subscript copy is in the middle of a paragraph.
<sup style="font-size: 10px; line-height: 0; vertical-align: 5px;">®</sup>
Our company was doing more and more email designs and running out of ways to test using non-enterprise accounts. I had been a free user of Litmus Builder for a long time and suggested getting an enterprise account. It was an easy sell. After that we even took on a new client that was using Eloqua for their emails and asked if we had an email testing service which was required by their company. It was great to say, "yes we use Litmus!"