Trying to help my designer by sort-of-marking-up the text

To paraphrase, you would like to send your designer the text but somehow tag it so that has the intended formats that you want. Fair to say?

InDesign has great control over formatting. I'd suggest reading this link first about how paragraph styles work: https://helpx.adobe.com/indesign/using/paragraph-character-styles.html

If the styles are already agreed upon and have appropriate names, something as simple as a tag at the start of a paragraph should be enough information to provide to the designer so they are aware of what paragraph style to apply. They can then apply the appropriate style when taking in the content and remove the human-readable tag to it once formatted.

Either that, or use PDF round-tripping. That is, let the designer prepare PDF proofs for you, and when you get them, highlight them using Acrobat's markup tools (link here https://helpx.adobe.com/acrobat/using/commenting-pdfs.html) so that using different coloured highlights can indicate to the designer what styles to apply.

@Nicholas38408755zs9x 

If the data has a constant - database-like structure - records with fields - your colleague can import it as a plain text, convert to a Table, select each column in the Table, apply either ParaStyle or CharStyle - or both - then convert back to text.

The way to convert - separator - and if Char or Para Style(s) should be used - would depend on if the data should be displayed as:

Date

Title

Venue

Descr

or

Date, Title, Venue, Descr

If first option - then separate ParaStyles should be applied to columns and column separator should be Paragraph:

If second option - then CharStyles should be applied to columns and column separator can be any character:

that can be later replaced with something "wider".

Find&Change - "#" -> ", ".

I can't address your question directly, but I can tell you that if your designer knows InDesign in any depth this really is not a problem.

InDesign is style-driven and it is easy to create a paragraph style that formats predictable patterns, such as a date, time, venue, description as long as there is some sort of consistency in the separation of the elements (which needn't be the same separator between elements, so long as it's always the same and does not appear before it is to be used as a style trigger). This can be accomplished using Nested Styles which are triggered by counting something, or GREP Styles which use pattern matching for mor complex constructions. Paragraph styles can be defined to work in cyclical fashion, too, so that a block of text can have styles A, B, C, A, B, C applied automatically ad infinitum

Once defined, presumably in your document template, one need only pre-load the styue before adding the text, or select the added text and apply the style to the entire block in one go.