Some thoughts re looking back before civil registrations for bdm ...
Back in the decades and decades before family history became armchairable via the internet, there were groups of people who transcribed the parish registers and put their findings into books, and these books were available to other groups, and via public libraries, .... so people in Australia who had migrant ancestors from say Cornwall, could look through those books, either at their local family history rooms, or at the main Public Libraries... And, in turn those looker uppers were able to transcribe local records (baptisms, burials, marriages, probates, newspaper cuttings, etc) and print off books and send those books to groups 'overseas'...
Alternatives to parish registers
* Newspapers - many are being digitised by various organisations around the globe. Some are freely available ... for example, the Trove resource from the National Library of Australia includes newspapers and various Government Gazettes .... births, engagements, marriages, deaths, funerals, probates, sequestrations, publicans licences, grants of land, selling of property, passenger lists on shipping, legal notices, etc
https://trove.nla.gov.au/ ... New Zealand also has similar resource:
https://paperspast.natlib.govt.nz/ and of course, the newspapers in the colonies carried news from the UK
including bdm type announcements. Both those resources are already listed on the respective RChat Resources Boards.
* RChat's resources boards are great, here's Cornwall's
https://www.rootschat.com/forum/cornwall-resources-offers/ https://www.rootschat.com/forum/index.php?topic=490272.0 - This live link is one you have provided, back in 2010. And, thanks to you, I was able to sort out one of my 'brickwalls' back then ...
JM