I have made a series of actions that allow me to add custom tags and to preview them with a custom css class.
I have this currently working only for my workflow, but since is something hard to find, let me know if more of you are interested in something like this, so I can modify my version to be more widely apealing
Example
In the following line, the custom tag is ¨m ¨ and has the class name “motion”, and with the styles it sets the text to bold and italic
¨m Any Text ¨
So the complete text beeing like
# Title
This is a line ¨m with a custom tag ¨
- This is a list item
While end up looking like this
Title
This is a line with a custom tag
- This is a list item
The tags could be set to anything if you know how to find it in Regex, or with a simple in and out descriptor.