I cant tell you how to do it as holy but I did heal Nef as disc last week if you want to try that.
In p1 I healed our frost DK kiter which didn't drain any mana at all, when adds died I had 100%.
The rest of the phase I pre-shielded the weak ones(not the rogues that feint, hunter that raptor strike, nor the tanks) and used barrier on the ranged during crackles which was about 5 shields each crackle(we run a 10man guild).
Between crackles my job was to get everyone up to full while keeping PoM jumping and rapture on CD to regain as much mana as possible (Prayer of Mending tracker and Ingela's Rapture are awesome addons to keep track of this). I usually top the raid off my using IF PoH on the group with most people that cant self heal(rogues with recuperate for instanceare exclused in this) and later used heal which most of the time was enough, if not GH or penance did wonders but drained more mana. The hardest thing in P1 is the Onyxia tail lash + crackle combo which can be quite nasty unless shielded properly.
I usually entered p2 with 70%+ mana, I pop Core of Ripeness(spirit use) and PI to top my group up by spamming heal which made me gain mana to full. The rest of p2 I tried to keep shield on myself to procc rapture while keeping PoM jumping and penance on CD, I did have to use lots of GH this phase as well as barrier during our single crackle(to make p3 easier). Once I got the hang of it I never had to use anything more than my trinket/PI macro and barrier during crackle to manage this phase.
At the start of P3 I popped a Potion of Concentration as well as fiend which almost got my mana back up to 100%. The rest of the phase I went back to pre-shielding the weak ones, using barrier, hymn, pain supp the tank and so on during each crackle to make it as easy as possible. Of course you use glyphed PoH with IF each time on as well as one on each group after crackle, gives quite a lot of healing for very low mana.
I havn't been healing very long as disc since my main spec is shadow but if you wanna progress, sometimes you need to stop relying on others and do it yourself