R – How to show a list of sites the current user can access

The Business Data Catalog is intended as the solution in this case. A BDC solution is crawlable out of the box.

Unfortunately the BDC requires the Enterprise licence for SharePoint, a hefty price increase.

Any other solution will require an html interface to the database either by including the data inside the SharePoint webpages themselves or a custom set of pages.

I have not tried using a custom search provider so I cannot make a comment on doing that.

It is easy enough to include searching on both content sources as OOB SharePoint provides the ability to distinguish between content sources and has the ability to create search scopes that allow rules based on content type or url to be created.

Not the most efficient solution to this problem, but you can iterate through all the site collections / webs in the web application and then call:

web.DoesUserHavePermissions(userID, SPBasePermissions.Open)

where "web" is a SPWeb object and userID is "DOMAIN\user.name" of the user you are checking access permissions for. Do it within a RWEP and make sure you dispose of your SPSite/SPWeb objects correctly.