But it wouldn't prevent the cross-realming issue which is my bone of contention with clustering pryd and excal: not the fact that they would prevent cross-realming, but that by preventing cross-realming you would be tied to one realm per account, or make you wait 8 hours before you could play your other realm-server alts.
I wouldn't cluster Prydwen and Excalibur, but rather Prydwen (EN), Carnac (FR) and Logres (DE), maybe even Deira (IT) and Cumbria (ES).
This would include the 3 smallest from each of the start servers plus maybe the 2 new ones so they can get a real RvR experience from the start.
The timer set by Mythic so you can't cross realm is a bit harsh, 4 hours or even 2 hours would have been enough, alot changes in 2 hours in RvR.
Edit:
Just saw the friday news on the site, it seems it will be Prydwen and Excalibur, but only with 1 hour of "downtime" and if you got chars on 2 different realms before the clustering is done you can still play both realms, it is only if you wanna start over you are "trapped" to play one realm only.