Mijiu is a type of Huangjiu, as is Shaoxing.
Mijiu is more like sake than mirin, fermented rice wine. Shaoxing is typically aged for many years and is always a reddish colour.
Mirin on the other hand usually has a lower alcohol content and is very sweet.
If you can find a Mijiu stating that it is 'Nong Tian' (extra sweet) then you probably could sub it in for mirin, but neither are difficult to find in any good Asian cuisine shop (in the UK certainly)... so I just keep bottles of both!
If you are referring to Chinese rice wine--Shao Xing is what my bottles say--it has no relationship to Mirin in any way and should not be substituted in my opinion. Shao Xing is more like an amontadillo sherry. Mirin is notably sweeter but also laced with astringency...
re: penthouse pup
This is the information I've found, from wikipedia:
"Mijiu - a clear, sweet Chinese rice wine/liqueur made from fermented glutinous rice, drunk as a beverage, used in cooking, or served as a dessert called jiuniang or laozao in southern China. Can be considered a category of huangjiu"
So, is mijiu a type of shao xing wine?
Wikipedia just doesn't work for me in general--entries are so often just plain wrong. For example, how can something be a "rice wine/liqueur"? The two categories are just not the same. Does your Mijiu have an alcohol content? If it's above 25% then it might well be a liqueur (most of which are in the 30% range.) Shao Xing that I have is 17%.
Why not just give it a shot and see what transpired? Here is a good link that will help: