By preview file do you mean rendering it as HTML and viewing it in a Drafts preview window?
If so you would need to modify the engine that generates the HTML, or its input/output, not the syntax which is what shows in the editor. One way to do this would be to use an action to preprocessor the file to convert the custom syntax to to the appropriate HTML, which is valid standard Markdown, and then pass that on to preview.
The syntax is the definition of a data/content format. A Drafts syntax definition is how that format is shown and interacted with in the Drafts editor only.
For example Taskpaper is a standard syntax in Drafts. The syntax definition determines how the elements are defined and determines the behaviour in the editor (e.g. checkbox cycling). In combination with the Drafts theme, this will determine aspects such as the colour and size of an element in the editor. So it is a combination of these two that determines the visual presentation; but only in the Drafts editor.
Drafts’ preview is HTML based. Some Markdown flavours are available for automated conversion to HTML (Standard/MMD/GFM), but you could utilise others. On i*OS, options are relatively limited but you could utilise existing alternate transformation engines using say Shortcuts and A-shell, or perhaps Pythonista. On Mac, with access to the command line you have more options, and should you be a Marked2 user, there is an option for custom pre processors - I use this to render custom substitutions I have built.for my website, which is effectively what is going on here… but it is Mac only.
In Drafts, actions can incorporate a Preview step, or use the HTMLPreview class to render HTML representations. This does appear to be the type of Preview originally referenced. Such actions can pass modified content to these Preview steps, which is what I suggested and what you repeated in your response. It is equivalent to the Marked2 pre processor option as the action modifies the content to include the necessary HTML (part of the standard Markdown spec is HTML compatibility) then transforms the content into preview able HTML.
Correct. The live preview is a self contained feature in Drafts.if a live preview is key, then as long as you are using it on Mac only, you could use Marked2 (Drafts integrated an external live preview with it) with the preprocessor option to do the substitution I suggested to place in an action.