A Zone 1-4 Travelcard Season would be valid for travel on Chiltern. As the free weekend travel offer terms mention having the Season on
Oyster▸ , it follows that a Season wholly for use in the Travelcard zones is eligible for the offer.
I see no problems with what autotank wishes to do. You can either purchase add-on tickets to/from Southall or Boundary Zone 4. Same price, same validity. No requirement to call when used in combination with a Season.
If you use an Off Peak Day
Single in the BZ4/Southall - Maidenhead direction, in combination with the Travelcard Season, you are okay to use any train off Paddington in the evening. The restriction code in that direction for the ticket from BZ4/Southall is W1 (
http://www.nationalrail.co.uk/pdfs/OffPeak.pdf#page=121), valid after 0930, no evening peak restrictions.
However, if you buy an Off Peak Day
Return with origin Maidenhead to BZ4/Southall, the restriction code is the infamous P7 (
http://www.nationalrail.co.uk/pdfs/OffPeak.pdf#page=86) and you cannot catch any trains on the 'barred' list. However, the only direct fast service, that calls at Maidenhead, on that list is the 1750 to Worcester SH. You'll also need to avoid the 1649 and 1851 to Oxford, changing at Slough.