Hi everyone,
I just bumped into this issue and came across this thread.
No fix before 4.27 is really not cool, this should be targeted at 4.26 and would almost deserve a 4.25 hotfix imho.
Anyway, i can confirm the workaround works well in 4.25, so enormous thanks to all of you 
As some extra info for anyone reading this, the bAllowCPUAccess is public, and, as i load my meshes at runtime, i tried to fix it in the code with:
ModelMesh->bAllowCPUAccess = true;
Unfortunately, this resulted in a crash in the packaged game when setting the modified static mesh on the spline mesh component:
NewSplineMeshComponent->SetStaticMesh(ModelMesh);
For anyone interested (or having an idea for a solution), here’s the stack
So bottom line, this workaround works well but atm you’ll have to modify in the editor all the static meshes you need for your splines.
Thanks again,
Cheers
Cedric