Nice, I just tested this ! This is great, thanks !
For the gnome/illusionist stuff, we are in a mixed state with these tweaks installed on EE :
Expanded Dual-Class Options
Alter Multiclass Restrictions -> Install options one and two (everyone can multiclass anything)
Remove Racial Restrictions for Single Classes
Alter Dual-class Restrictions -> Allow non-humans to dual-class
Remove Racial Restrictions for Kits
The gnome multi-class options are with Mage instead of Illusionist, but we can't select the Mage single class.
I think we should have 2 options (for EE at least it seems doable) :
Mage single class accessible + Mage in multi-class options : k_m_g.2da should be modified and be the same as the other races (add line "1 0"). Maybe in 'mgsrcreq.2da', in the GNOME column, all the lines should be set to 1 for consistency, but I don't see any difference with only Mage + Illusionist set to 1.
Mage single class inaccessible + Illusionist in multi-class options : mgsrcreq.2da GNOME column should only have 1 set for Illusionist. However k_m_g.2da should not have the line "1 0" because it causes Illusionist to be duplicated (Mage seems to be replaced by Illusionist in position 0 with this conf in 'mgsrcreq.2da', so yeah some things are definitely hardcoded...).
I even managed to allow any specialist mage in multi-class options (see attachment).
What seems to be impossible is : Mage single class accessible + Specialist mage in multi-class options.