[RPG] Can the Heat Metal spell target a set of Dragon Scale Mail made from metallic dragon scales

RAW there really is only one option to quickly take off armor: have someone help you.

Getting Into and Out of Armor

The time it takes to don or doff armor depends on the armor’s category.

Don. This is the time it takes to put on armor. You benefit from the armor's AC only if you take the full time to don the suit o f armor.

Doff. This is the time it takes to take off armor. If you have help, reduce this time by half.

The fastest a PC could shed heavy armor with help would be in 2 and a half minutes or 25 rounds of combat.

RAW options to avoid the damage

1. Run away. As I interpret the spell you would still need to be in its range to use the bonus action attack. Running out of range would be difficult to achieve since the spell attack is a bonus action allowing the caster to use both their actions to follow you. However if allies impede the movement of the caster or your move speed is greater it could be accomplished in one round.
2. Deal Damage. Every time the magic user takes damage while concentrating on a spell they are forced to make a CON save to keep that spell up. If everyone hits the wizard odds are pretty high that he will fail one of those CON saves.

Houserule option

Let the PC take it off with a standard action, but doing so damages the armor, preventing them from wearing it again until they have it repaired.

Musings on Heat metal

There is no requirement for a PC or NPC whose gear is affected by heat metal to react taking off their armor. The stipulation of the spell is to throw it if they can, donning and doffing seems to be a bit more than all that and as such I would interpret it to mean weapons should be thrown, but armor can be kept on. I know 2d8 bonus action damage sounds like a lot, but dice only damage has a way of only coming out to be around the average most of the time, while 9 damage a round without an attack roll is pretty nice, it is probably a better idea for the party to focus-fire on the wizard to end it rather than someone in heavy armor taking their armor off and seriously lowering their AC for the rest of the fight.

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.