Poet's Pen life sustain options

I can't think of anything but life on hit/gmp/ frenzy( or Barrage) that gives life sustain using these things. There's the unique jewel Watchers Eye Prismatic Jewel, but getting fire damage leeched as life is random and there are none for sale.

How can this be viable in any sustained damage intake situation?

I run thief's torment and barrage with GMP. You dont have any other options really. You could sacrifice a link for life on hit which i would recommend you do. If you can push guardians/bosses up against a wall with projectile weakness etc you can basically just spam kinetic blast with gmp.

Life leech as a caster is always rather limited. You can use Warlord's Mark as a curse, giving you a steady 2% life leech even with spells. Plus you generate endurance charges as an added bonus.