I am currently able to edit my pioneers in the unit design window. This allows me to create all sorts of odd machinations.
Side effects include:
- units with no wages
- units that are hardlocked to group-sized units (3 members)
- units that are able to produce outposts and settlements
The problem is this piece of xml
<CanBeDesigned>1</CanBeDesigned>
which makes them trainable but also "designable". To remove the ability of designing them, you need to add
<IsUniqueButBuildable>1</IsUniqueButBuildable>
to each pioneer. This removes them from the design menu but keeps them in the training menu.
If you are purposefully keeping them in the unit designer so people can produce riding pioneers and so on, you should consider giving them their own modeltype so the player cannot equip them with armor or weaponry. Another option would be to automatically unlock a "stronger" pioneer when mounts are unlocked.