[Druid Question] Spellcasting while shapeshifted

As per title, except for the spell(s) that explicitly says can be casted while shapeshifted, is there a way to permit other spells to do so? A gem, a unique, a node of some kind?

I understand that could be potentially game breaking, but I can really see a build based around an always wyvern form that exploits elemental spell damage, without having to revert to human form

I don't imagine so. Although the main issue with your idea there is that Wyvern form would be using a Talisman which can only roll attack damage modifiers, leaving your spells without a key source of scaling from a staff or other spellcasting foci.