importance demarginalising

A question on X validated gave me minor thought fodder for my crisp pre-dawn run in Warwick the other week: if one wants to use importance sampling for a variable Y that has no closed form density, but can be expressed as the transform (marginal) of a vector of variables with closed form densities, then, for Monte Carlo approximations, the problem can be reformulated as the computation of an integral of a transform of the vector itself and the importance ratio is given by the ratio of the true density of the vector over the density of the simulated vector. No Jacobian involved.