A 90 min Cloak, even for a single attacker with multiple targets in multiple guilds (and a lot of stam, of course), to be able to cover himself for a day, takes a lot of Potions. It would also mean the attacker being the only target for repercussions, otherwise it gets multiplied by the amount of possible targets in the attacking guild.
Assuming a four to six man guild, it's entirely possible that a single player is the only one who is viably a target.
Quite apart from the fact everyone sleeps at some point or other
People in this very thread have done 24 hour stands to ensure GvG victories. You're assuming that people don't do anything and everything to win, and that's patently not true.
and kits can still be guessed at, extra stats can be seen and calculated looking at buffs active etc.
Defenders are already in a corner when it comes to GvG. Now we're going to make it even more difficult for them? This is a step in the very wrong direction. No guild will initiate on a cloaked player, how is it reasonable to hold the defenders to a higher standard when getting guilds to hit back is a problem as things stand?
It can be a short term help, for sure, how it would be a massive long range and worthwhile win-button, that I don't see, tbh.
In terms of the ebb and flow of GvG,
both sides have to log off sooner or later. Every hour a guild is prevented from hitting back is a huge win for the other side.
And I agree with RJEM as far as you're taking it with relics in mind, one agressive measure is the same as any other.
The relative cost of empowering a relic to 10% is, at minimum, 50 to 100 times greater than the average cost of cloak so far. That's like EA175 being just as powerful as EW1500, only worse.