Is there a liferay function or utility, or a 3rd party one, to export a complete portal instance from one liferay portal server and import it into another?
It would have to allow the portal instance to be manually allocated to a shard when it is being imported.

The complexity for doing this, and the reason we don't just use an SQL script, is that the surrogate keys for records associated with a portal instance are not guarenteed to be unique between portal servers. This means the utility would have to identify clashes and alter the incoming primary keys and associated foreign keys to be unique and to properly use Liferay's primary key allocation mechanism.

I'm facing the possibility of having to write this and would prefer to find one already out there.

Not that I know of. The closest thing is the staging feature but it doesn't copy portal settings.

FYI the approach we are taking for this functionality is as follows:

1) Manually create a destination portal instance with the basic meta data including user roles and permissions
2) Use LDAP to transfer users and groups from the old portal instance to the new one
3) Use LAR files to transfer all the content

Anyone forsee any problems with this approach?

I see in JIRA that some work has been done to automate portal setting transfer: http://issues.liferay.com/browse/LPS-26599 but obviously there are "complexities".

With our manual process we will need to work out how to duplicate user groups and roles at the target in such a way that users being ldapped across (I invented a new verb!) will connect to the correct groups and roles.