This exact question was answered just this Friday in the most recent FAQ posting on paizo's site.
Wild armor and other transforming armor: When I use a wild armor and gain the armor’s benefits, what restrictions, if any, apply to me? In general, when I transform with a polymorph effect and some of my gear melds into the form, what restrictions do I have for melding with large amounts of heavy gear? What about other types of transforming armor?
If you were in medium or heavy load from encumbrance before transforming, you continue to take those penalties in your melded form. Otherwise, ignore the weight of melded items and calculate your encumbrance in your polymorphed form entirely based on non-melded items. When wearing melded armor and shields, if you gain no benefit from the melded armor, you still count as wearing an armor of that type, but you do not suffer its armor check penalty, movement speed reduction, or arcane spell failure chance. If you do gain any benefits (as with the wild property), then you do suffer the armor check penalty, movement speed reduction, and arcane spell failure chance. This also applies to all other situations where you or an armor transform: you always count as wearing an armor of that type, and if you gain any benefit at all from the armor (such as mistmail), you apply the armor check penalty, movement speed reduction, and arcane spell failure chance.
It can be found HERE
Dungeon Master Guide, page 165
Dragon Scale Mail
Armor (scale mail), very rare (requires attunement)
While wearing this armor, you gain a +1 bonus to AC, you have advantage on saving throws against the
Frightful Presence and breath weapons of dragons,
and you have resistance to one damage type that is
determined by the kind of dragon that provided the
scales (see the table).
Additionally, you can focus your senses as an action
to magically discern the distance and direction to the
closest dragon within 30 miles of you that is of the same
type as the armor. This special action can't be used
gain until the next dawn.
The armor has also base features of a scale armor, which means:
- it will grant AC of 14 + 1(magic) + Dex modifier (max 2)
- disadvantage of stealth
If it is not enough, you could make this item legendary, by increasing the armor bonus by 1. Then it would grant 16 + dex mod (max 2).
If your character relies on stealth and high dexterity (16+) you could get feat Medium Armored Master to negate armor's disadvantage and increase max dex cap to 3.
Anyway it appears that you have missed the most important Rule 0.
The D&D rules help you and the other players have
a good time, but the rules aren't in charge. You're the
DM, and you are in charge of the game. That said, your
goal isn't to slaughter the adventurers but to create a
campaign world that revolves around their actions and
decisions, and to keep your players coming back for
more! If you're lucky, the events of your campaign will
echo in the memories of your players long after the final
game session is concluded.
Dungeon Master's Guide, page 4
Your players may dislike it but, as Game Master you can create anything that you would like to. Although you shouldn't ruin the fun for everybody, as it would be simply immature.
Druids have problems with metal equipment? Why won't you make a full plate armor made from draconic bones, skin of Tarrasque, scales of Bahamut/Tiamat or pure diamonds. Anything is possible, use your imagination! That is what role playing is about.
You should try to find out the consequences yourself, however if you find it difficult, there are possible consequences, which I often use:
- The weight is increased or decreased, maybe even multiple times in comparision to the original.
- Change of the value.
- Unique materials are needed to maintain the armor's performance. Blood, grease, saliva, extract from unique species.
- (for magic armor)The items work only, when all pieces of the set are gathered.
The more positive aspects may be based on already existing ones or the source of the armor.
Examples:
- Granite breastplate weights 3 times than normally (60 pounds), costs 300 gp and has a disadvantage on stealth tests.
- Shell full plate armor costs 2000 gp, and weights 50 pounds and must be regularly treated with Plesiosaurus' mucus.
- Diamond full plate armor is priceless, gives immunity to critical hits and resistance to physical, non-magic damage.
- Hide made from Sahuagins gives advantage for underwater stealth tests.
Best Answer
Let me reply to your question with a related question.
I think the answer to this question is pretty clear. According to the Rules As Written, nothing bad happens. There are no rules about this situation, so the ritual has no effect and there are no consequences for your cleric.
However, that doesn't actually mean nothing bad happens. Your cleric has offended Pelor, and it is the DM's responsibility to tell you what consequences your cleric will suffer. The lack of rules just means the DM has to make something up.
In the same way, if a druid is forced to wear metal armor, it is the DM's responsibility to tell you what consequences that brings. As Dale pointed out, there are no rules for this situation -- but that doesn't mean nothing bad happens, it just means the DM makes something up.
As for what the DM should make up: the DM might start by looking at previous editions of D&D, such as this quote from D&D 3.5: "A druid who wears prohibited armor or carries a prohibited shield is unable to cast druid spells or use any of her supernatural or spell-like class abilities while doing so and for 24 hours thereafter."
I would also propose that the druid's animal companion tries to prevent him from equipping the armor, and while the druid is wearing the armor the animal companion does not recognize the druid (it treats the druid as a stranger).