|Think about Loose Coupling|
If it works for you go for it! Interviewing is hell.
The point I was trying to make that in my experience your method would cull some very good candidates at an early stage. For me personally, another day wasted with idiots in interviews is worth catching those people.
One of the many problems with looking for community contributions is that some people work under IP restrictions that prevent them from contributing. This was true for myself for several years.
BTW, the cheating problem is the same
What I was trying to say was:
Work online == plagurist. No false positives (if your work is online, you copied it). False negatives (just because it wasn't online doesn't mean you didn't copy it).
Not online == bad coder. False positives (you can not be online and be a good coder). False negatives (online presence misses lots of aspects important to being a good coder).
If I've misinterpreted your point, apologies.