I think a ticket for an include macro option to preserve tabs is also
The include directive does not remove tabs by default. Only if you set the indent attribute on the include directive and set the tabsize document attribute will tabs be expanded, but that's intended for use with source code, not tsv.
Thanks Dan - I was clearly reading old asciidoc doc (thus my mention of the `include1` macro above).
All great to know, I'll embedding the tab character in the document as separator for now and will watch the issue above (thanks for creating that).