I don't think there is a problem with the odds - if the HIP unit is allowed to remain HIP until attack declarations in CC and opts to do so, I don't think it will be allowed to attack anyway.
I think you misunderstood my point about calculating CC odds. Whether or not the hidden unit decides to
attack in CC is irrelevant. Because the prisoner attacks first, and because it is entitled to attack any or all units in the Location as long as it also attacks its Guard, it is (normally) entitled to know what the Strength Factors of all opposing units in the Location are, concealed or otherwise, in order to calculate the CC odds.
But wait, there's more!
Given the presence of a hidden unit in the Location, ambush
ought to be a possibility too. (Granted the prisoner would receive a +1 drm for being Lax, (and absent any other drm) the net drm of the Guard's side would be -1 [+1 broken, -2 concealed]).
Ambush is important because it may influence what the prisoner decides to attack in CC. Moreover, if the prisoner succeeds in ambushing its opponents, concealment would be lost, leading to the possibility of Melee.
All this is to say that it's important to establish whether A11.19 would apply retroactively should a situation such as yours occur. Have you sent the question to Perry?