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Arthur,
will do. Since I am searching for a good idea why our group would want to enter Moria, the "Quest for the Arm" might be a good reason to do so. Particularly because next on their journey, the player-heroes need to search for old lore at the library in the White Towers. Maybe they stumble over some sort of respective ancient writing that provides a hint for such a spell and store it with their fellowship journal, just for gathering intelligence for their patron Elrond (unaware that one day it will be them that might need that Intel)... or maybe that note even mentions such witchcraft being witnessed by a long ago expeditionary elven / dwarf commando into the land of Mordor itself, providing a cause for the group to "walk into Mordor" once one of the player-heroes lost that arm... I also don't rule out that the Witchking might even be the dark sorcerer that attempts to corrupt the group by offering that arm recover-spell as reward for some evil deal, creating a wonderful moral dilemma... opportunities for an epic quest - with many side-adventures in Moria / on the way to Mordor / towards the Witchking's realm - abound!
However it'll play out in the end, I'll post a summary then.

I definitely would talk about the losing an arm-part with the players first. If they are not okay with that, let them find an alternative.
Since there are no rules for losing limbs im TOR, it is a pure story-wise decision, and for that players should be onboard.

Fellow TOR Loremasters,
For any ideas / critique / funny comments I should be most grateful. And even while I would love to find a creative solution for the hero to continue his heroic life adventuring forth and fighting the shadow, I absolutely do not want to bend the Tolkien canon to a degree that makes the whole game ridiculous by inventing overly fantastic deeds like planting a new arm from scratch on the respective hero...

Loremasters,
thank you for the thoughtful comments, very much appreciated!
Knowing the players quite well and understanding that they very much enjoy sudden twists, dilemmas and extreme challenges to their characters, I think I will go with the "Session 0" approach Linklite suggested and then creating an epic quest for an arm along the line Arthur suggested (i.e. a new arm created and attached by (dark) sorcery that has special abilities but comes with a hefty price, likely some sort of severe shadow scar or that special shadow path). I am very sure that this is the approach our group would enjoy.
It is really great to learn from your input, so the "swarm intelligence" provided by this forum is indeed invaluable, viewing the posted topic from different angles / LM perspectives.
Our Group member that plays the Lindon Elb character is the born hero, always volunteering for the most dangerous tasks and taking on the most dangerous enemies in battle, so I guess it will eventually be him that volunteers to put his arm into the monster's snout. Coincidentally, it is also him who currently paints the (rather big) 1:32 miniature of that terrible monster, not knowing when and in which adventure it later will play a decisive role in the life of his character... especially since that bite wound that severed his arm off probably is a "grievous injury" (p. 101 CRB), potentially killing him if no healing attempts succeed.
As I said, I'll keep you informed here with our experience and how things turn out and develop, but that will still take a long time since first I want them to finish the TotLL campaign so that they can grow for what awaits them in "Raiding the Obsidian Keep", which I will probably place somewhere at the Sea of Rhun.