Determine Closedate Depending On A Metatable And This Closedate Will Be Used In A Query

Hi, This is a diffcult issue to explain. I hope to make my problem
clear to you.

SITUATION
I'm building A SLA Query for a customer. This customer has an awkward
way to determine the SLA results ;-) Depending on a category which is
stored in a headertable (Requests) a field and logic is determined how
to get a proper Close_Date. This Close_date can be the closedate of
the request. It is also possible that the close_date is a certain
detail record (Request_lines). Also It is possible that this
close_date is the ordered_date of a certain line_item.

DONE SO FAR
I have created a metatable with rules per category. With this rule i
would like to determine a close_date. This close_date will be used as
parameter in a function from which i determine the total minutes how
long this request has endured.

GOAL
I want to create something like this:
SELECT id, getdiffSLA(@StartDate, GetCloseDate(some parameters))
FROM...

I've created the function 'GetCloseDate()' in which i want to
determine the closedate. In this function i want to determine the
rule. Execute the proper logic and get the date.

This is what i'm trying to do in a function (this is one rule of many)
:

......
ELSE IF @iRuleNo = 2 --SAD_A_REQUEST_LINE.CLOSE_DATE
BEGIN
......
......
SET @vcSQL = N' SELECT @max = MAX(Close_date)
FROM samis.dbo.SAD_A_REQUEST_LINES
WHERE Parent_Quote = ''' + @vcNumberPRGN + ''' AND Part_No In ('+
@vcIN_String + ')'

Exec sp_executesql @vcsql, N'@Max datetime OUTPUT', @max OUTPUT

SET @dCloseDate = @max

ELSE IF....
.....

THE PROBLEM
This doesn't work and yeah i know i can't use exec /sp_executesql in a
function. But if i try this in a stored procedure i can't use this in
a Query.
SELECT id, storedprocedure FROM ... Doesn't work, also.

An option is that i could create a cursor, loop every record and call
the stored procedure, get the close_date, execute my SLA calculation
function, store the result in a temptable, use this temptable in the
query <pffff>.

But the table consists of 200000 records (with a detailtable) and
performance is a issue. It's used for loading a datawarehouse and not
in a OLTP system so a bit slow performance is allowed but not to much.

SO how do i do this without using a cursor???

Greetz

Hennie