After some trial and error... here is the answer for anyone running into this thread
url.rewrite-once = (
"^/dw2/doku.php/(.+)?" => "/dw2/doku.php/$1?do=export_xhtml"
)
I am not 100% sure that this is completely correct but it appears to deliver the desired result - appending do=export_xhtml to the supplied url.