Technically, it doesn't matter what you queue as. You can queue as DPS if you want--as long as you kill the boss as Holy. The game only checks your spec at the time the roll occurs, not what you queued as.
This is correct. Queuing and selected roles have no impact on what gear is available to you. If you're still not sure what's going on with this, here's another way to think about each roll:
The gear that you can get directly from the boss is determined by your spec at the instant the game calculates your roll immediately after a boss death. Since you can't change specs in combat, what this means in practice is that your loot selection depends on your spec at the beginning of the fight (barring a bug). By the same token (see what I did there?), the selection of gear you can receive from a coin roll is determined by your spec at the time you trigger the roll
. This means it is possible to switch specs immediately after a boss kill in time to use a coin roll on a different selection of loot.
To maximize your chances at Holy-specific items, you would indeed have to be Holy during the relevant fights and remain Holy while using the coins. However, as suggested above, you can still take advantage of the coin rolls while playing Ret or Prot for the fights themselves (as far as I know, anyway)...you just have to remember to switch before the roll if you're doing it this way. If you're there exclusively for xmog gear, why not just queue DPS and try your hand at a shockadin playstyle? The worst that might happen is a vote kick if the group is awful and DPS is the issue ;)