First - Registration selection

As the 3000 has no midi message that can select registrations then you would not be able to do this via any external device, so pedals wouldn't be any use either.

Trouble is I've downloaded the 740 manual and I can't find anything in there either that would give this ability.

So now I'm a bit confused. I can't see how you could remotely select Registrations on either the 740 or the 3000. So what messages did to use on the 740 to remotely select registrations?

Secondly voice selection

You can certainly configure the 3000 to "listen" to the patch change data coming in from the PA50; you map L / R1 / R2 on the 3000 to listen to midi channels 1 / 2 / 3. The problem with this is that the PA50 will send the patch changes that it is using to select its own voices and this may not be what you want to select on the 3000. You need some way to send independant patch changes to the 3000, and this is the sort of thing that you can do with master keyboards, but I don't know if you can do this with the PA50.