Well after a bit of reading through the code for Blosxom I’ve tracked down the right place to put a temporary hack in to create the required
Here’s the diff from the original Blosxom code for anyone interested:
436c436 < rss story <item>\n <title>$title</title>\n <link>$url/$yr/$mo_num/$da#$fn </link>\n<description>$body</description>\n </item>\n --- > rss story <item>\n <title>$title</title>\n <link>$url/$yr/$mo_num/$da#$fn </link>\n<description>$body</description>\n <pubDate>$dw, $da $mo $yr $ti:00 +0 100</pubDate>\n </item>\n