passing filter options with the --convert-to CLI switch to soffice?

I'd like to be able to use the convenient --convert-to CLI switch on
soffice to convert from Word to PDF. However, I'd like that conversion
to create tagged PDFs, and the default for UseTaggedPDF is false.

A number of PDF export options are listed at
https://wiki.openoffice.org/wiki/API/Tutorials/PDF_export, and after
reading that I tried,

$ soffice --convert-to pdf:writer_pdf_Export:UseTaggedPDF=true --outdir /tmp myfile.docx

But still got untagged PDF in the output file.

Can anyone see what I'm doing wrong?

Thanks in advance!

Bill