The general idea is intuitive, if the faction queue is long, then allow player to transform into opposite faction to play battleground. On a more technical level, this, with considerations and limitations could become a complicated system.
Feasibility below:
1) if (AshenBattleground.getAshenBattlegroundTime() > maximumWaitTime()) then
Spawn Mercenary
2) Faction specific achievements are removed; this can cause a problem to develop.
3) Chat functionality reworked; you have to communicate with Alliance players, could cause issues.
4) Race functionality reworked: are there are functional constraints, namely, will the racial change, e.g. Orc changing into Human, will they have Every man for themselves racial, or an Orc racial that will give away they are mercenary.
5) item functionality; this could change since the system has to believe the character has a temporary faction change. What if you have a Horde specific trinket, then change to Alliance, will this trinket also change, or will it not be useable? This will cause problems to code.
6) the masks that Blizzard use to fool for the opposite faction example is a representitive way of conveying this idea quite simple. However, the functionality of the system will have to be changed, and constraints will have to be implemented based on the non-functionality intention.
7) Blizzards intention is to disguise the player to discreetly pass them off as their own faction. Thus, each of these will have to be considered. Perhaps an Alliance inspects a Horde who is disguised as Alliance, and sees a trinket that can only be equipped from the Horde, or using a racial or passive that is only found from the opposite faction. There is a lot to consider.
8) Achievements will also have to change completely. If you are Horde changing to Alliance who has done For the Horde, these achievements will also have to be changed to match the identity, in particular, change to For the Alliance if disguised as the Alliance faction,, this is a non-functional constraint, as part of Blizzards "discreet" ability to pass as the opposite faction.
9) Game Design; the look and feel of the character will change. What if the character owns Horde-specific arena gear that has faction specific colours and only owned by Horde players. If they faction transfer in a battleground, what will their character look like? Will the Horde-specific gear and colours have to be changed as a result? These are considerations to keep this discreet.
This is not an exhaustive list but only just a few considerations of implementing this system. This will entirely depend on How Blizzard intend to develop the system.