This is a good topic to bring up again since they have yet to implement a feature like this for PvP healing:
Healing should have diminishing returns when consecutive heals on the same target are cast by any source other than the player. Self-heals are not affected by diminishing returns for NON HEALING specs.
Basic Implementation:
A stacking debuff that stacks up to 100 times with each stack reducing all healing received by a percentage that varies based on the class and spec affected, because a healing DR would have a lot less effect on a tank spec or class with high defensive abilities than it would on a squishy DPS spec with limited defense, but it would nonetheless be a very easy and fair way to tune heals. You shift it from buffing/nerfing the healing done by healers to limiting the amount of healing a particular class/spec can receive.
How it could work:
Each time a player is healed, the debuff increases stacks in proportion to the amount of healing received. The ratio of stacks to healing received can and should be tuned. Self-heals of a non-healing spec are never affected by the debuff. Healing-specs self-heals will be affected by the DR.
For every 1 second that the a player is not healed by any source other than self-heals, the debuff stacks tick down exponentially (-1%, -2%, -4%, -8%, -16%, 32%, 64% etc). Exponential means that after 5 sec, the player sheds about 30% of the stack, and after 7 sec a player will completely shed the debuff entirely...but consecutive heals within 4 sec or so will not have significant impact on the stacks, thus allowing players to maintain pressure while allowing skilled healers/dps to disengage and run the clock down on the debuff. It makes pvp a lot more tactical.
Tweaks and Abilities:
This debuff opens up new possibilities for healing abilities that temporarily bypass the DR, either by allowing heals to ignore it OR by causing heals not to add stacks to it. DPS specs can also have some of their special abilities increase the healing DR debuff or prevent it from ticking down. In both cases, it should be an on-use ability with a reasonably long CD of 2 mins or more and effect that lasts 4-8 sec or less.
Benefits:
If this was implemented as outlined here and tuned as recommended per class/spec, it would make pvp a lot more tactical and allow for a lot more ebb-and-flow without any player getting to be "immortal". It naturally scales with group size, because in larger groups the healer will be healing more targets for a lower amount, thus building up debuff stacks at a lower rate. In smaller groups the debuff builds up a lot quicker, thus restricting the generally OP nature of heals in 2v2 and 3v3.