How about:
The first installer install OP+, and the *structure* needed to add models, ie, the directory trick that's being talked about here.
The second installer not install OP+, but just the models that will be produced to support OP+, meaning that it would fill the structure created by the first installer. So long as the default models in use now match up in size to the models that replace them, I'm hoping that the siz file won't have to be adversely affected.
Other than HD size requirements expanding, this should allow peaceful intergration of OP+, OP+ with models, maintain the newfound ease of installation, and allow them both to play on the same dynaverse/gsa without problem.
A simple suggestion,
Holocat.