I'm working on a mod that does almost exactly what you want, and I intend to release it just as soon as patch 1.1 goes live.

How the mod works:

Pioneers require a resource called "Settlers".

This resource is acquired two different ways.

1. You receive "settlers" when you level up a city. (enough for one pioneer per level of the city)

2. You receive "settlers" by reaching certain fame milestones. (enough for one pioneer per fame milestone)

What is left to do on the mod:

1. Finish adjusting the fame milestones for settlers

2. Double check and correct an issue where the resources given and used are doubled when the mod is installed exclusively to the mod folder (I think this issue has more to do with me fiddling with the core files rather than an issue with the mod.)

3. Adjust the size of the settler resource graphic. I'm using the settler wagon from Master of Magic, and it is a tad too small.

So far in my testing, the AI handles it fine. Meaning, when it hits those milestones or levels a city, it immediately builds a pioneer like it normally would.

Of course, the AI wouldn't understand it needs to do those things to get the pioneers, but fortunately the AI is already predisposed to try to reach those goals.

You could just make it a non stored resource instead of stored so that they can build only one at a time instead of just once per level.  I foresee the same problem listed from Parrotmath otherwise.  Having it as a non stored resource you could vastly reduce the amount of pioneers to only 1 per level 1 city and then only have towns supply more than one of the stored resource.  Interestingly enough you could actually force the AI to only build units within a fortress using this same method of non stored resources by making all units require a non stored resource that Fortress City Hubs only make.

Well It would be great if you could have limited pioneers and maybe a different type of unit to settle outposts, would be really nice. But its a shame like you said that the AI wouldnt use them

I haven't noticed that in my implementation. They get settlers at level 3, 4 and 5 as well, also through gaining fame from questing (15 fame, 50 fame, etc).