Group by entry type in custom export filter

Before escalating this to a feature request or bug, I want to make sure I’m not doing something wrong.

I’m creating a custom export filter, and i would like the entries to by grouped by their type (as in “Article”, "Booklet, “TechReport”, etc…).

If I use:
\begingroup{entrytype}{\b NEW ENTRY TYPE:} \endgroup{entrytype}

the text is never inserted (just a a double check, if I replace “entrytype” by, for example, “year”, everything works, so it’s not an issue with my filter definition.
In truth, “entrytype” is not strictly a bibtex filed… so maybe this is not strictly a bug, but then it is a missing feature. Or, should I used a different field name to represent the entry type?