I've decided to calculate the absolute maximum amount of HP that can be healed with a single spell.

First, some assumptions:
The spell used is Rafiereed. (Feireed doesn't use the normal healing formula. Other healing spells are weaker.)
The character using the spell is not confused. (Otherwise, we need to consider Hand of Kindness on a superboss.)
Everything is within legal limits. (No hacking characters beyond them.)
The caster gets lucky with the RNG.

Anyway, here's the calculation:
Rafiereed heals 45-135 naturally. We'll suppose the caster gets lucky and rolls 135 points of healing.
22 Piety (possible for a Gnome) adds +15 to that amount (before multipliers) yielding 150 healing.

Now, multipliers:
First, there is the Rainbow Charm (Ibag Tower 16F+ rare drop), which grants a 200% bonus to all elements.
Now, we apply Alchemy:
The maximum AP an ore can have is apparently 50 AP.
We then factor in High Purity Synthesis used by a level 15,139 (level cap) Alchemist. This boosts the AP by (1.5 * sqrt(15,139 - 25))%, which is 184.4%. (The ingame display is 284.4% because it adds the 100% baseline. Other High Mastery skills don't work this way.) That yields 142.0 AP. (By the way, the Alchemist must be a Human, Elf, Dwarf, Gnome, or Hotlet to reach this level.)
200 + 142 * 2 = 484 (assuming there isn't some limit I don't know about).
So, ignoring Cleric Spell Power Up, our Gnome can, if lucky, heal 876 to the party. (Note that this does not beat the Hand of Kindness baseline of 1999 on max HP characters.)

Now, we need to factor in Cleric Spell Power Up +1. Our Gnome Cleric can reach a maximum level of 16,485, and Cleric Spell Power Up +1 (CSPU+1) has a base of 10%, so it reaches 10 * sqrt(16485 - 25), which is a 1282.9% boost. So, the resulting healing ends up being 12114 points.

12114 points of healing is the maximum possible heal on a party member. Note that HP caps at 9999, so spells like Feireed can't reach this level.

The minimum heal at this level of power is 4845, and the average is 8479.

At this level, Rafelima averages 2907, so Hand of Kindness suddenly seems rather useless at such extreme levels. The problem, of course, is getting there; reaching this level requires nearly 4.3 billion experience points.

Edit: Fixed name of stat (there's no Wisdom stat in this game)

Testing replying since this thread seems to be missing from the topic list.

Such testing should always go hand in hand with in-game testing in my opinion. i t is also interesting to see what will happen with masteries if to decrease exp. requirements to level up.

Also I can't understand quoted part (142 * 2), where do you get the 2 multiplier? From what I saw, you get average AP of 2 materials (and not their sum), though I might be wrong with enchanting.

When forging, raising an elemental attack or defense modifier gives you 2 points per AP spent. Therefore, 142 AP should let you raise the Holy attack bonus by 284%, so the Rainbow Charm's bonus would increase from 200% to 484%.

You are confusing forging with transmutation. Transmutation is what takes two materials and averages the materials' AP. Forging is what lets you improve your equipment.