The DB initialisationis done after the siteaccess, so in principle using the additional parameters should override it (not tested myself though)
About the "driver": you should use the same as the ini settings (the aliases), so mysql, ezmysql for example are synonyms and usable as long as the corresponding ini settings are set (look at the default site.ini shipped)
@Geoff: what I would do is create a fake admin siteaccess for the remote instance, and run updatesearchindexsolr.php with the --siteaccess parameter. But be aware that if the binary files are not readable from the filesystem, they will not be indexed (if you use this feature).
Of course it would be better to consolidate the DB parameters, even though they will most likely work.
