What could work would be to give the doviello a unique camp improvement that grows into the yaranga, and give both lumbermills and normal camps the same upgrade improvement for everyone else. This upgrade should only happen on ancient forests when the tile is owned by a FoL civ. That way, conquered/built lumbermills (and camps) will automatically become this theoretical improvement once the forest upgrades. Unfortunately, this will need extra coding, because as far as I can tell the .xml file only allows for improvement upgrades based on civ (eg doviello, kurio, grigori), not religion or feature type.
On-topic, this is what I tend to use the religions for:
The Order: awesome second hero, stay-at-home pacifist-type high priests, unique building that acts as a second courthouse (eliminate your maintenance with a couple of law mana), terribly awfully dull soundtrack
Ashen Veil: combatty fun normal priests who like to burn things, cool high priest summon, great heroes (squee! mershabber!), arguably the best holy city shrine (stigmata promo), savants can get disciple promotions then upgrade to mages (not all that great, but useful if you have spiritual trait), farms are irrigated with BLOOD, creepycool soundtrack
Runes: mithril golem avatar hero, normal priest has a buff that ISN'T removed by combat (in your face, order!), gold!, normal priest can use earth's bosom to level mountains (difficult to do; you need to be dwarven or jotnar, have a kuriotate airship, or get lucky with a captured dwarf priest), good unique civic, world wonder can help set up great engineer farming, soldiers of kilmorph to build dwarven mines (seriously, convert for a few dozen turns if only to build these guys), decent soundtrack
Leaves: ancient forests (pretty!), best looking farms and windmills (more pretty!), second hero is good for terraforming, normal priest can summon kitties, more health and happiness, ok soundtrack
Octopus Overlords: already been said, but tower of complacency for no unhappiness (combine with thousand slums/gardens of amatheon), undead units with water walking, archmage hero, high priests can utterly destroy enemy navies, soundtrack isn't great
Empyrean: Chalid (!), and high priests with AoE spell, normal priests have a spell to dispel magic AND see invisible units, temple provides science boost, essential shrine (especially if fighting d'tesh, sidar, or anyone following CoE), makes alignment more good (might just push you into hell terrain immunity), probably the best soundtrack
Council of Esus: archmage hero, surprise attacks, shadow riders, shrine makes your workers effectively invulnerable (at the very least)
White Hand: terrorist priests (when their summons die, they explode the surrounding land into ice), dragon hero, temples and shrine give extra production and commerce, slowly turns all your lands into glacier, and the AI has absolutely no idea that this religion is generally a BAD IDEA for them
Question time: the reason I didn't mention the ability for council of esus units to spread their religion without dying, is that it is not a useful point in RiFE (unlike other FFH modmods). This is because religions will automatically spread to cities even if they already have another religion present, eventually causing every city to have every religion. May I ask where this awesome piece of code is located? I really want to steal it and put it into every other CIV4 mod I have