I found out that it is in fact possible.
First, create a new named calculation in the DSV using a case statement (for example, call it [Gross Bal Over 90]):
CASE
WHEN [Days Since DOS] > 90 THEN [Gross Balance]
ELSE 0
END
Then, the calculated measure would simply be:
Sum([Gross Bal Over 90])/Sum([Gross Balance])
You can then make [Gross Bal Over 90] invisible to the user, keeping a cleaner look.