Using it as a resource, I suppose you could create a unit buildable by anyone with no pre-req EXCEPT the Merc resource, with a build cost of 1

(and make it a National Unit, or otherwise limited access number), that way it only minimally interrupts the production of other things. Or make a spell which is only castable in a town with access to the Merc resource which will hire a Hippus Merc. And of course Hippus themselves would not be able to access this unit.
I think that having the Hippus able to build a UB at a fairly early tech (Stirrups or Horseback Riding) which provides them with a single Merc resource is a good approach. Then they can trade off 1 for each city they have built, so it is limited, but not TOO limited, and they will be able to always have access to the resource without having to find it on the map.
Leaders are set to a certain threshold for when they will be willing to trade with you based off the Pleased/Annoyed/whatever attitude marks. I am not sure if you could use Python to hard-code around this since that is all established only in XML, but it sounds possible. In that case, you allow the AI to trade Merc resource at ANY attitude level (as long as they are willing to communicate of course),
AND you hard code it so that they value the merc resource at 5 gold per turn (or some appropriate value).
Then, the AI will offer you 5 Gold per Turn to grant them Merc's if you are playing as Hippus, but if you counter offer with almost anything else they will consider the Merc resource equal to you offering 5 GPT.
I do believe that having the Merc units be STR 0 + X Merc Affinity is the perfect solution, because then you cannot just build your Merc units and then cancel the trade (and thus stop paying for them) without loss of the units. And that way if you manage to acquire multiple merc Resources, you get better Mercs.