In 2.3, with the introduction of the lag-free casting (queued casting), this opens the option of macroing your spells/heals with certain Librams for each individual heal. Before in 2.2 and below, swapping librams while casting caused noticable lag time and even temporary frame loss if spammed. with the "queued casting" as they call it, your spells are now queued after clicking at a certain point in the cast time, in which before casts did not queue, but only seemed liked they queued if using a stopcasting macro. What this means is that very minute operations (switching librams) can take place during the split second that the queue is up for your heal/spell cast with zero lag time.

What this means for Paladins is that we can carry around several different librams at once, each with different effects for a certain kind of heals (Libram of Absolute Truth for Holy Light only, Libram of Light for Flash of Light only), and macro them to be equipped upon the click of that spell, and continue to be swapped in with no lag time, successfully proccing that Libram effect. Here's some examples:

/equip Libram of Mending
/cast Holy Light(Rank 11)
with,
/equip Libram of Light (Souls Redeemed if the raid has BoL)
/cast Flash of Light(Rank 7)

or even this....

/equip Libram of Absolute Truth
/cast Holy Light(Rank 11)
with,
/equip Libram of Light (Souls Redeemed if the raid has BoL)
/cast Flash of Light(Rank 7)

or for those of you who picked up Libram of Grace from Fankriss in AQ40...

/equip Libram of Grace
/cast Cleanse

These macros can effectively be used for any spec of Paladin, Holy, Protection, or Retribution, you just have to know which Libram works with which spells of course.

Druids and Shamans can do this too with their relics, potentionally creating a new use for Relics that might have been inferior to another.

I was under the impression that weapon or gear swaps incurred a global cool down on everything. (at least this is how it works for me on live) I don't think the stop casting change changes this at all.

I tested it on the ptr, and swapping relics don't cause a global cooldown, only the heal does. And the casting changes do effect this, the reasons I gave were simply speculation. When testing it on live I got frequent lag while switching, and even random error/spell is already being cast messages, which is not apparent in 2.3.

Yep, you get a global cooldown, you're supposed to swap the libram after the spell cast and not before:

#showtooltip flash of light
/stopcasting
/cast flash of light
/equip...

the only change for 2.3 is that you will no longer need to write the /stopcasting part...

Does this work with instant casts?

As in, is this viable for prot paladins?

I don't think it'd work for prot libram swapping since they trigger immediately, and you are swapping before the spell is finished casting. You'd have to swap first, so holyshield/ judgments would come 1.5 seconds after you hit the key.

Edit:
I don't know if I'd bother, but you could potentially gain upwards of 7 mp5 by having 2 HL macros of your standard rank, one with equiping Libram of Mending, and the other with Libram of Absolute Truth. Landing the Mending one every ~10 seconds.

It does indeed cause a global while in combat. Which means if you start a heal and then cancel (common, duh), you're going to be running into globals everywhere and really impair yourself. I don't recommend it, although it looks good on paper and when you're fooling around with the macros out of combat and how nifty they are.