/.../ I can't think of a real reason to make that function configurable, unless you're worried about performance of randomness vs. quality of randomness.
As Nilsson mentioned, the TLS apps have set that function themselves. I see several examples of that now, both in Roxen and in Protocol.HTTP.Query when it sets up a client side TLS connection. So the de-facto interface seems to be that you should do it in your own app before the SSL connection is established.
I can't see the point with forcing the apps to do that explicitly, so a decent default value for SSL.context.random would be nice. It wouldn't produce any problem with the existing apps either.