Yes, growing rootstock in a container makes it easier to graft--no bending over. But, from what I've heard, the most vigorous rose generally takes over--one wouldn't expect scions of multiple varieties to co-exist year after year.

Multi- grafted roses are frequently sold here in Australia and I have grafted many varieties onto the one bush in the past for fun and it's easy to do but I no longer have multi- grafted roses in my garden as they simply don't look very good or neat and I don't need space that desperately.