Can you use both? I'm not sure how OSD is accessing the custom table, but let's say you key on computer name. You current query might be:
SELECT Name, [MAC Address] FROM Inv_Custom_OSD WHERE <whatever>
You could modify this to be:
SELECT vc.Name, tcpip.[MAC Address]
FROM vComputer vc
JOIN Inv_AeX_AC_TCPIP tcpip ON tcpip._ResourceGuid=vc.Guid
WHERE vc.Name IN (SELECT Name FROM Inv_Custom_OSD)
--The above SQL returns name and MAC address for computers from Altiris, if they exist in both Altiris and the custom OSD table
UNION
--..and combines it..
SELECT osd.Name, osd.[MAC Address] FROM Inv_Custom_OSD
WHERE osd.Name NOT IN (
SELECT vc.Name
FROM vComputer vc
JOIN Inv_AeX_AC_TCPIP tcpip ON tcpip._ResourceGuid=vc.Guid
WHERE tcpip.[MAC Address] IS NOT NULL)
--with this SQL, which returns Name and MAC address for computers in the custom OSD table who do not have a MAC address recorded in Altiris
I don't have the custom table, so I haven't run this, but it should return a single list of computers, with MAC address from Altiris if there, otherwise from the custom static table.