Since it's been nearly two years now and I've more or less given up hope of getting Shard of the Scale, I picked up Scarab Brooch from Viscidus last night:

Scarab Brooch
Binds when picked up
Unique
Trinket
Requires Level 60
Use: Your magical heals provide the target with a shield that absorbs damage equal to 15% of the amount healed for 30 sec.

The way I'm thinking here is that it's effectively 15% more healing, only better because it's preemptive after the first shield.

Given that I would otherwise be using Mindtap Talisman in its place the math seems sound. I generally heal the MT on Patch, who is taking more or less constant damage. If I use this trinket and downrank one rank of HT for the duration I save more mana in theory:

Mindtap = 11mp5 for 6min -- 792mana

Downrank HT for 10 heals = 600mana saved x 2

...but only if I'm casting all 10 of the heals possible while the effect is running.

The question is... anyone else tried this on Patch, or other fights even? Is the possible saving worth the guaranteed mana from Mindtap?

Scarab brooch is essentially a long cooldown long duration trinket that benefits big heals more than it benefits small ones (unlike pretty much all other activated trinkets that just give flat +healing). As such, it's best used in high healing output scenarios and doesn't sound too bad on patchwerk if you don't have access to other epic trinkets.

Still, activated healing trinkets are a bit of a hassle so you shouldn't use it if it lowers your concentration level notisably.

It would seem from the tooltip description overhealing also reduces the effectiveness of this trinket.

It doesn't (tested).

Even if you cast a heal on someone at full HP they'll still get a shield. Only heals able to crit give the shield though: healing touch, regrowth initial cast, whipper root tuber(!) unless changed. Rejuvenation does not give shields. Haven't tested with HP potions.

This is too narrow a question to be its own thread. We have a thread specificaly about healing on Patchwerk.