Be it magnesium oxide (supplements) or magnesium hydroxide (Milk of Magnesia) apparently the body will use either for it's nutritional needs.
A quick google found the following nugget about magnesium hydroxide (MOM version, though not necessarily MOM in the study).
Magnesium may be beneficial for bladder problems in women, especially common disturbances in bladder control and the sense of “urgency.” A double-blind trial found that women who took 350 mg of magnesium hydroxide (providing 147 mg elemental magnesium) twice daily for four weeks, had better bladder control and fewer symptoms than women who took a placebo.12http://www.mothernature.com/Library/Ency/Index.cfm/Id/2879002So if you're using MOM, you'd need to take that into account in adding up your daily magnesium intake totals.
My bottle of Mom says that 1200mg is the dose. Using the numbers above and a little math, I get 504 mgs of elemental magnesium from a single dose of MOM.
The US RDA is around 400mg, unless you are pregnant. Many argue this is too low, that the RDA should be raised.
But it is always safe to avoid continuous high doses of minerals...