Subtotals within union query

I have 3 separate tables to track inventory: Count for actual inventory counts, Build which uses/decreases inventory, and PurchaseOrder which adds to inventory. I've built a union query to bring all those records together for use in a DataPilot in Calc.

SELECT "Part", "Date" AS "EntryDate", CONCAT( CONCAT( TO_CHAR( "Date", 'YYYY/MM/DD' ), ' ' ), "Label" ) AS "DateLabel", "Qty" FROM
(
SELECT 'Count' AS "Label", "PartCountDateTime" AS "Date", "PartID" AS "Part", "PartCountQty" AS "Qty" FROM "PartCount" AS "A"
UNION
SELECT 'Build' AS "Label", "BuildDate" AS "Date", "PartID" AS "Part", - "BuildQtyAdjusted" AS "Qty" FROM "Build" AS "A"
UNION
SELECT 'Purchase Order' AS "Label", "PODate" AS "Date", "PartID" AS "Part", "POQty" AS "Qty" FROM "PurchaseOrder" AS "A"
)
ORDER BY "Part", "DateLabel"

And I can get a useful/current inventory quantity by adding a date condition:

SELECT "Part", CONCAT( CONCAT( TO_CHAR( "Date", 'YYYY/MM/DD' ), ' ' ), "Label" ) AS "LabelDate", "Qty" FROM
(
SELECT 'Count' AS "Label", "PartCountDateTime" AS "Date", "PartID" AS "Part", "PartCountQty" AS "Qty" FROM "PartCount" AS "A" WHERE "A"."Date" = ( SELECT MAX( "B"."PartCountDateTime" ) FROM "PartCount" AS "B" WHERE "B"."PartID" = "A"."PartID" )
UNION
SELECT 'Build' AS "Label", "BuildDate" AS "Date", "PartID" AS "Part", - "BuildQtyAdjusted" AS "Qty" FROM "Build" AS "A" WHERE "BuildDate" >= ( SELECT MAX( "PartCountDateTime" ) FROM "PartCount" AS "B" WHERE "B"."PartID" = "A"."PartID" )
UNION
SELECT 'Purchase Order' AS "Label", "PODate" AS "Date", "PartID" AS "Part", "POQty" AS "Qty" FROM "PurchaseOrder" AS "A" WHERE "PODate" >= ( SELECT MAX( "PartCountDateTime" ) FROM "PartCount" AS "B" WHERE "B"."PartID" = "A"."PartID" )
)
ORDER BY "Part", "LabelDate"

I give all this background to hopefully avoid fully defining each table and giving sample data for each. However, if that's needed to get a solution.... Anyway, on with the question.

Sample result generated by first query:

Part   EntryDate                  DateLabel                               Qty
-----  ------------                  ------------                                ----
0       02/17/09 12:00 AM   2009/02/17 Count                   1234
0       02/23/09 12:00 AM   2009/02/23 Build                     -236
0       02/25/09 12:00 AM   2009/02/25 Purchase Order   234
1       02/17/09 12:00 AM   2009/02/17 Count                   3432
1       02/20/09 12:00 AM   2009/02/20 Count                   3500
1       02/25/09 12:00 AM   2009/02/25 Build                     -345
1       02/25/09 12:00 AM   2009/02/25 Purchase Order   343
1       02/26/09 12:00 AM   2009/02/26 Count                   3400
1       02/26/09 12:00 AM   2009/02/26 Purchase Order    34

What I need added, is a (sub)total after each Purchase Order, of the quantities, from the most recent Count to the Purchase Order.

Giving a result set of:

Part   EntryDate                  DateLabel                               Qty
-----  ------------                  ------------                                ----
0       02/17/09 12:00 AM   2009/02/17 Count                   1234
0       02/23/09 12:00 AM   2009/02/23 Build                     -236
0       02/25/09 12:00 AM   2009/02/25 Purchase Order   234
0       02/25/09 12:00 AM   2009/02/25 OverShortTtl        1232
1       02/17/09 12:00 AM   2009/02/17 Count                   3432
1       02/20/09 12:00 AM   2009/02/20 Count                   3500
1       02/25/09 12:00 AM   2009/02/25 Build                     -345
1       02/25/09 12:00 AM   2009/02/25 Purchase Order   343
1       02/25/09 12:00 AM   2009/02/25 OverShortTtl        3498
1       02/26/09 12:00 AM   2009/02/26 Count                   3400
1       02/26/09 12:00 AM   2009/02/26 Purchase Order    34
1       02/26/09 12:00 AM   2009/02/26 OverShortTtl        3434

If possible, I'd like to do it just with queries. Multiple queries are fine, I'd just like to avoid scripts and code if possible. If that's no possible, then so be it.