(Tribal) Migration Modding

[Wasturji]

Hi all,

I asked a question about 2 in the quick questions thread, but as I search for an answer on my own and try messing around with migration more generally, I'm finding that modding the mechanics surrounding it is not straightforward, so I'm wondering if anyone else has tried or has advice.

I'm trying to do a few things with migration:
1. Create an omen effect reducing migration cost;
2. Edit the unit created upon migration start;
3. Change the rules surrounding migration, allowing migration under siege in certain circumstances;
4. (Reach) Allow migrants to 'carry' treasures for certain benefits.

I'm working on 1 at the moment, and I'm getting a "none" under Omen effect. Could have something to do with naming.
In deities\01_heptadic.txt, under the deity, I have

omen = { start_migration_cost_modifier = omen_start_migration_cost_modifier_svalue }

and in script_values\omen_svalues.txt I have

omen_start_migration_cost_modifier_svalue = -.15

I'm not sure if the _svalue part is necessary but adding/subtracting it seemed to have no effect.

I was thinking that modifiers can't just be changed with omens like that, but global_pop_conversion_speed_modifier seems to change with
omen_global_pop_conversion_speed_modifier_svalue; e.g.
Zoroaster.