Something like this?

WITH

MEMBER

[Measures].[Custom] AS

AVG(EXCEPT([Dimension].[HIERARCHY].CURRENTMEMBER.CHILDREN, {[Dimension].[HIERARCHY].CURRENTMEMBER.FIRSTCHILD,[Dimension].[HIERARCHY].CURRENTMEMBER.LASTCHILD }), [Measures].[Your Measure])

SELECT

{

[Measures].[Custom]

} ON 1

FROM [Your Cube]

WHERE (

[Dimension].[HIERARCHY].Some_Member)