This question builds on the great answers I got on an earlier question:
Can one extend the functionality of PDF, CDF, FindDistributionParameters etc in Mathematica?<
The following page contains some tips on enabling custom distributions (i.e. written from scratch without TransformedDisribution or ProbabilityDistribution) for use in CopulaDistribution, RandomVariate, etc: https://mathematica.stackexchange.com/questions/20067/efficient-generation-of-random-variates-from-a-copula-distribution/26169#26169