option 1 (general usage, not supported by Evernote today) you could use HTML5 data attributes
<span data-nodeID="vdsijfewo329ds"><en-note>This is a note</en-note></span>
option 2 you could try <figure>
- <figcaption>
HTML5 pair:
<figure>
<en-note>This is a note</en-note>
<figcaption style="display: none">vdsijfewo329ds</figcaption>
</figure>
option 3 class attribute (this was the reason data-* attributes were created, this could be the last resort option
<span class="en-note vdsijfewo329ds"><en-note>This is a note</en-note></span>