For what it's worth my opinion is that IF in the end the two community Squeak Source servers are merged (and I'm not personally convinced that is the best way to go, but I'm not strongly opposed to it either), THEN it makes sense to use the hostname source.squeak.org for it.
Now, today, I'm quite happy to setup sourcetest.squeak.org. In fact, I've just done that:
+sourcetest.squeak.org:130.92.65.106
Use it if you like, ignore it if you prefer.
I think we need to first concentrate on speaking with those people that have an interest in the two servers and seeing if anyone has any objections to the merge. If for any reason we decide against the merge then I think it would be a mistake to have assigned the source.squeak.org hostname to the external packages server. And as Cees has said you can go ahead and do everything you need to do test and confirm the setup, even use it, using the sourcetest hostname for now and when and if the merge occurs changing the hostname will be a minor job compared to the rest of it.
Ken