Pretty selfexplanatory question from the title...
The new Stoneborn Pact keystone mastery, im interested if it does or doesn't work with the % Healing and Shielding power increasing effects the likes of which we have on items such as 
Ardent Censer, 
Mikael's Crucible and 
Redemption? Or is it perhaps a different healing effect that isn't affected by these item effects?
It does.
It doesn't work on yourself, the tooltip for it explicitly states "Your crowd control effects brand enemy champions with a earthen rune for 4 seconds. OTHER ally champions who attack branded enemies heal for 5 + 2.5% of your maximum health over 2 seconds (halved if you are ranged)."
So no, you yourself don't gain any benefits from attacking a branded enemy (Otherwise this would just end up being a support's Grasp of the Undying) but your other allies do heal off of it...
About the "halved if you are ranged part"... would be nice to know which part of it is halved exactly... My guess it's the "5 + 2.5 % of your maximum health" that is halved for ranged champions, since it doesn't seem like the duration of the heal over time on allies is shorter for ranged users that apply this effect.
I haven't tested it, but I assume that the healing doesn't get increased by those items. Instead, the healing received can be increased by Spirit Visage, Runic Armour etc.
And does it work on yourself?
yes heres me playing stoneborn pact miss fortune support with ardent censer to prove it does https://gfycat.com/NimbleEasygoingGnu
Yes it does. An Amumu with Rilay's and W toggled on will make you heal ridiculous ammounts of health.
not sure about those but it should at least work with self healing boosts like 
but i guess it's time to pull out the 10k hp sion
Stoneborn stats that you heal them so yes it should benefit from those items. To further this fact, Stoneborn procs Athenes.
However I am still trying to figure out if Frozen Heart procs StoneBorn as its technically a movement impaired item and it synergies with Torment.
It all comes down to whether healing received by attacking a marked champion is registered as Stoneborn Pact's user's healing or self-healing. This would also determine whose "healing" stat is bolstered in the end-game screen.
