[eevan]

OK, I have tried. Created a custom column named #original.
Filled it with date, and saved EPUB to disk. The resulting meta tag in the EPUB is a bit impractical for use, because it's content is a long JSON string with a full description of a custom column, not just the date value. It is just one line of course, I have broken it just for better display in a code block.

<meta name="calibre:user_metadata:#original" content="{&quot;link_column&quot;:
&quot;value&quot;, &quot;is_multiple2&quot;: {}, 
&quot;search_terms&quot;: [&quot;#original&quot;], 
&quot;is_csp&quot;: false, &quot;rec_index&quot;: 22,
 &quot;column&quot;: &quot;value&quot;, 
&quot;datatype&quot;: &quot;datetime&quot;, 
&quot;label&quot;: &quot;original&quot;, 
&quot;name&quot;: &quot;Originaly published&quot;, 
&quot;kind&quot;: &quot;field&quot;, 
&quot;is_custom&quot;: true, &quot;is_category&quot;: false,
&quot;#extra#&quot;: null, &quot;is_editable&quot;: true, 
&quot;display&quot;: {&quot;description&quot;: 
&quot;Date of original publishing&quot;, &quot;date_format&quot;: 
&quot;dd.MM.yyyy.&quot;}, &quot;is_multiple&quot;: null, 
&quot;table&quot;: &quot;custom_column_4&quot;, 
&quot;category_sort&quot;: &quot;value&quot;, 
&quot;#value#&quot;: null, &quot;colnum&quot;: 4}"/>

But thanks anyway for your help!