You could achieve this using the Axis function in MDX. Here is an example in AdventureWorks:

WITH MEMBER Measures.PercentOfAxis AS

[Measures].[Internet Sales Amount]

/

SUM(AXIS(1), [Measures].[Internet Sales Amount]), FORMAT_STRING="Percent"

SELECT {

[Measures].[Internet Sales Amount]

,Measures.PercentOfAxis

}

ON 0,

{[Date].[Calendar Year].AllMembers} ON 1

FROM [Adventure Works]

In this example the Axis(1) value happens to be AllMembers of Calendar year so a percentage is calculated per row for each year. Whatever you put on the 1 Axis will be calculated in the same way (provided the dimension is connected to the Internet Sales Amount measure).

I hope this helps.