All my current databases have the prefix "domain_._" but trying to enter this in the installation form and the fullstop just gets stripped out and so it cannot connect to the database.

I could remove the fullstop form my domains settings, but that would mean renaming all the other databases (and associated programmes) currently using this domain which is far from ideal.

Is there a workaround for this?

You could try just try installing it then overriding the database name in the config file. I don't know why the installer strips . So what you can try is to manually install SS. All the installer does is check install requirements then create the mysite/_config.php file and the .htaccess file. You can do this yourself. If you have installed SS before / on a local webserver just upload the .htaccess file from that to the root folder of your silverstripe installation. Then copy the mysite/_config.php file of your installed version to new site and edit this file with your correct database details