Hi Tom.
I'm afraid that multipath isn't something I have much time for (you have to draw the line somewhere....).
The multipath implementation in md was written by Ingo Molnar some years ago and apparently left to rot. I have tried to make sure it didn't rot too much while maintaining other parts of md, but I haven't been in a postition to improve it. I don't use multipath myself, don't know the fine details or what the important issues are, and don't have any hardware that would allow me to test it.
It is my understanding that the 'dm' based multipath is seeing active development and it should be quite usable and reliable, but again - I have never used it and so cannot comment directly.
I will not be putting any effort into multipath on md. However if you or anyone else would like to work on the code, either in the kernel or in mdadm, I would be happy to review any changes and get them included in mdadm or Linux as appropriate.
Sorry I cannot be more helpful.