How would you go about having WSS search index content that's inside a webpart/pulled from an external source and presented in a SPGridView?
You probably already know this, but if you go to Site Settings -> Search Visibility you will see a section stating:
This site contains fine-grained permissions. Specify the sites ASPX page indexing behavior: ...
If you choose "Always index all ASPX pages on this site" it should index the content in your web part, but only as the crawler sees it, so security trimming would not apply. It is basically a web crawl and not a SharePoint content crawl.
I know you said WSS, but in MOSS you might be able to take this one step further if the above did not work out and use a web site search on your SharePoint site. I have done plenty of web site searches (it does not work perfectly), but have not tried to explicitly do a web site search on a SharePoint site so I'm not certain this will work.
Lars (who co-wrote "Inside the Index and Search Engines: MOSS 2007") is pretty active on StackOverflow so maybe he'll chime in.
Also in MOSS: Have the data you are displaying available as for instance a web service / page in a different site also. In Moss you can add Federated Search locations to be included in the Search results.