You said it: the value of a field is a "single atomic sequence". All you can do is apply a single character style to it to change its aspect from the surrounding paragraph.
I experimented with a pasted DDE link where the source text had several character styles applied to it (and also contained various fields). The field value is a string of characters stripped of any stylistic variation (similar to unformatted text).
The only way I found to keep original formatting was to Insert
>Section
with Link option (without DDE).
I emphasise the word keep to mean you have a mirror of the original file. If you want to be able to change formatting, you must "unprotect" the section, but next time you Tools
>Update
>Update All
, Writer retrieves the content of the file and all your formatting changes are lost.
I could not succeed in using the DDE link of a section (the command I entered was ineffective and I didn't search why).
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