Hi @Michael Davidovich 
We are also developing a library of REST endpoints and have this same question - to which we can't seem to find the answer! Unless I am mistaken (I am new to REST in IRIS) the suggestion made by @Sergei Shutov only works for 'manually created' REST services and not for Open API spec first? As the URL Map for spec first is stored in the .disp class which is automatically generated by the import of the swagger.json so cannot be modified to split the endpoints into different .impl classes?
I would be very interested to know what conclusion you came to for how to structure your REST app so it is scalable. 
Many thanks, 
Amy

What a great suite of tools! Thanks!