You could try the tags for required city lvl for this. Look at the various buildings to find an semi appropriate tag and see if it works with base units. However, I am not sure if this will work as no other unit works like this. You could also tie the pioneer to a building of some kind which requires the city level you want. However, it may not work since a pioneer is a starting unit type for every race.

Now, I will point out that this kind of change may have undesirable effects from AI opponents. The AI who has a desire to expand normally spends its resources on pioneers and expansion bases. Without this, the AI will probably spend all of its resources trying to kill you.

I believe you can directly reference the cityhub internalname in your BuildingRequirement.

Good point. Instead, add a Special Resource Type to the affected cityhub levels. Then reference and demand that resource type in the pioneer unit. See diplomacy units and diplomacy improvements as an example.

Edit: This may not work for accessories, ie it might not work for custom-made pioneers.