It's only "allowing" herself if she has a choice. I doubt if she does. She can't just give herself new powers without paying a cost that would be unacceptable to her - Lara wants to be in charge, not to be in debt/in thrall to some other power (like a Fallen in a Denarian coin). Working with Odin/Monoc Securities is just a business relationship, she can still be in charge of her own.
Thomas was really damaged, enough to be non-functional/near-death.
If Thomas's Hunger is at least as strong as hers, she probably couldn't transfer enough energy to get him back to normal without reducing herself to non-functional/near-death, even if it was a 100% efficient transfer.
And it might not be efficient, because Thomas was too out of it to really feed - and the White Court probably aren't set up to give energy away (as somebody else pointed out).
Do we know that this would actually work?
The kind of energy the White Court feed on seems more personal and less "magic physics" than what Harry uses, it might not be able to just be stored in an inanimate battery.
Otherwise Harry could just have done a spell to "top up" Thomas... with time to do thaumaturgy, he could probably draw much more energy than Lara could provide. Since Lara didn't suggest that, I think it probably takes feeding on a living being.
I don't think the Whampires can just get out of their inherent limitations that way, any more than the most powerful Rampires or Blampires could avoid drinking blood.
She wouldn't have to sell her soul. She'd have to trade like she did with Mab, or buy like she did with Monoc.
As for the backup reservoir, I'm not saying it'd be easy to make on the fly, which is the only way Harry ever makes anything. But given that we've seen several sorts of magic power, as well as soul power and love power, be permanently or extendedly imbued into objects, it stands to reason that there should be a way to artificially store lust power as well.
Theoretically it wouldn't have to be that much different than what Harry already did. The belt buckle wasn't kinetic energy. It was a magic reservoir.
I touched my left hand to my belt buckle and whispered, “Fortius.”
Power rushed into the pit of my stomach, a sudden tide of hot, living energy, nitrous for the body, mind, and soul. Raw life radiated out into my bones, running riot through my limbs. My confusion and weariness and pain vanished as swiftly as darkness before the sunrise.
This was no simple adrenaline boost, either, though that was a part of it. Call it chi or mana or one of thousand other names for it—it was pure magic, the very essence of life energy itself. It poured into me from the reservoir I’d created in the silver of the buckle. My heart suddenly overflowed with excitement, my thoughts with hope, confidence, and eager anticipation, and if I had a personal soundtrack to my life it would have been playing Ode to Joy while a stadium of Harry fans did the wave. It was all I could do to stop myself from bursting into laughter or song.
If Harry, an amateur wizard in his twenties with little to no formal training, can manage to do that, then I don't see a problem with an older, wiser with wizard or mage or Monoc contractor doing something similar, but with a focus on storing a supply of lust energy. It could just take a little from each feeding to keep it topped off.
And honestly, I could see where she might take it a step further. Give a white pearl or diamond or opal to every Raith that will be their personal reservoir, and link them together so her larger gem would get a slice from each feeding. Then she could strengthen her own people and gain power from them.
It'd just be combining things we've already seen in the series.
As for it having to be from a living being, we know magic comes from living beings, but can be stored in inanimate objects. And we've seen a wamp feed on energy from another now, sans the sex, so we know it doesn't have to be just that for Raiths. Even if there's an inherent loss in storing and tapping, it'd be better than just shrugging at the idea that she'd find herself physically challenged to the point that she'd be tapped out and helpless.
