The holdings don't neccessarily have anything to do with high-level source holdings but rather 'well-established' source domains. In a way, this is conceding what has been a long standing idea about source domains, i.e. that they are insubstanial, ephemereal, if you will.
I think I have an idea that could do this.
High MSP grants virtual trade holdings in a province equal to the MSP - 4. These holdings can only be created by Source domains. In all other respects, they act as trade holdings. These holding represent wizardly services for hire, tutelage, sage services, alchemical, and/or other wizardly/scholarly services within the province.