Well, this isn't exactly what I wanted to do since it doesn't seem to be as clean as I would like but...

I took the statement from the function and put it directly in my SELECT as a Correlated Subquery:

SELECT t.HPROPERTY AS HPROP,

CASE WHEN t.dtleasefrom BETWEEN s.begdate AND s.enddate AND t.dtrenewdate IS NULL

AND ISNULL(p.dtapply, t.dtsigndate) IN

(SELECT CalendarDate

FROM (SELECT CONVERT(SMALLDATETIME, CONVERT(VARCHAR(4), YEAR(DATEADD(yy, -1, begdate)))

+ '0901') +

N3.NUM * 100 +

N2.NUM * 10 +

N1.NUM AS CalendarDate,

begdate

FROM (SELECT 0 AS NUM UNION ALL

SELECT 1 UNION ALL

SELECT 2 UNION ALL

SELECT 3 UNION ALL

SELECT 4 UNION ALL

SELECT 5 UNION ALL

SELECT 6 UNION ALL

SELECT 7 UNION ALL

SELECT 8 UNION ALL

SELECT 9) AS N1,

(SELECT 0 AS NUM UNION ALL

SELECT 1 UNION ALL

SELECT 2 UNION ALL

SELECT 3 UNION ALL

SELECT 4 UNION ALL

SELECT 5 UNION ALL

SELECT 6 UNION ALL

SELECT 7 UNION ALL

SELECT 8 UNION ALL

SELECT 9) AS N2,

(SELECT 0 AS NUM UNION ALL

SELECT 1 UNION ALL

SELECT 2 UNION ALL

SELECT 3) AS N3,

SEASONS

WHERE (hprop = t.HPROPERTY AND shortdesc = @season)) GenCalendar

WHERE CalendarDate < CONVERT(SMALLDATETIME, CONVERT(VARCHAR(4), YEAR(begdate)) + '0901'))

AND ISNULL(l.renewal_transfer, 'No') <> 'Yes'

THEN 'New Leases'

WHEN t.dtleaseto > s.enddate AND t.dtleasefrom <= s.enddate AND t.dtrenewdate

IS NOT NULL AND t.dtrenewdate IN (SELECT *

FROM dbo.GetCalendarTable(@Season,553))

THEN 'Renewals'

This allows me to use the outer table's t.HPROPERTY value as I need.

Of course, if someone knows how to make the other way work, I'm all eyes.