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Never mind; I got it working!
You can overload any texture file that is part of the livery. It just happens to be that there are not yet any modded Fulcrum pilots in the DCS liveries.

For anybody searching for the solution:
Open the model that you want to mod in the modelviewer2. Regenerate LUA: Alt+Ctrl+Shift+G Read the LUA to see the names you need to map. Without this, I could not have found out that `Pilot_MIG29_Helmet_Visor` would be the name for `Pilot_MIG29_Glass`.


name = "PRFS" --order = 0 livery = { -- Built-in mirror renderer {"MiG29_Cpt_mirror_L", 0, "mirrors0", true}; {"MiG29_Cpt_mirror_M", 0, "mirrors1", true}; {"MiG29_Cpt_mirror_R", 0, "mirrors2", true}; {"mig29a_ext01", 0, "mig29a_tex1_vvs", false}; {"mig29a_ext02", 0, "mig29a_tex2_vvs", false}; {"mig29a_ext03", 0, "mig29a_tex3_vvs", false}; {"mig29a_ext04", 0, "mig29a_tex4_vvs", false}; {"mig29a_ext05", 0, "mig29a_tex5_vvs", false}; {"mig29a_ext05_BN", 0, "mig29a_tex5_vvs", false}; {"mig29a_ext05_BN", DECAL, "mig29a_ext_bn1_vvs", false}; {"mig29a_ext06", 0, "mig29a_tex6_vvs", false}; {"mig29a_ext07", 0, "mig29a_tex7_vvs", false}; {"mig29a_ext08", 0, "mig29a_tex8_vvs", false}; -- Plastic {"mig29a_ext09", 0, "mig29a_ext09", false}; -- Pilot {"Pilot_MIG29_Helmet_Details", 0, "Pilot_MIG29_Helmet_Details", false}; {"Pilot_MIG29_Body", 0, "Pilot_MIG29_Body", false}; {"Pilot_MIG29_Helmet_Visor", 0, "Pilot_MIG29_Glass", false}; {"Pilot_MIG29_Helmet_Shell", 0, "Pilot_MIG29_Helmet_Shell", false}; }

Apparently it is a MiG-29 Fulcrum specific problem, as I compared it to the Tomcat and that does allow me to change the pilot/helmet.
 
 

Updated and tried ECW-slotting again after about a month. Slotting problem seems fixed.
There still is a process eating 100% of a single core, but it does not block the rendering.
off-topic: I have had troubles with the launcher since its existence giving me errors and (for example) saying I'm not logged in, but when I start it anyway the game shows I'm logged in and all modules are available.

With the adjusted graphical settings the stutter is gone; even when flying through the guts of the Phantom:
Solution by @Rapierarch:
PS Closest I have ever gotten to a Tomcat (not close enough to reproduce):