On the server WoW-Pwnage, if you make a nightelf on arena-52 (pvp) or illu (custom) you get shadowmeld, ofc. But there is a bug where if you shadowmeld in a different form then it will swap you and your spells back to regular form (Or in druids case, cat form) and allow use of (almost)ALL your spells, while still having benefits. To do this, all you have to do is shapeshift and take a step forward.
Example for priests: Go shadowform. You get a new toolbar. If you use shadowmeld and step forward, your regular form toolbars come back, you can cast from all 3 schools, and you have your shadowform buff still up and all of its talent related benefits.
Example for rdr00ds: Go tree form. Once again, new toolbar (I know, its obvious). Now shadowmeld and take a step forward, and shadowmeld. You go back to your caster toolbar, but you stay in tree with its bonus armor and healing. But now you can cast ALL your balance spells.
Example for boomkins: Go boomkin. New bar. Shadowmeld, and step forward. This is by far the weirdest one, and the most helpful. It swaps you to your cat form toolbar. You can use all your balance and resto spells, but you also can use prowl (hell yeah!) pounce (boomkin stun, lol) and dash. This is very helpful, and its pretty hard to get caught doing unless they SS you casting a resto spell.
I have not tested warriors yet, and they might be interesting to do this for (overpower in berserk stance). I have only tested this on the server WoW-Pwnage (Not their blizzlike realm), so feel to post other servers this works on.
TL;DR: On the server wow-pwnage if you shadowmeld in a form you go back to regular spells while staying in the form.