When an opposing captain tells me in advance that she is defaulting a court, I decide which two players should take the default with strategic considerations in mind.
I might decide to use those two slots to qualify players for the post-season, as OP hopes to do.
I might decide not to list a player who is nursing and injury and who might later wish to be deleted from the roster. My listing her might mean she could no longer get a refund of her registration fees.
I do not believe the opposing captain has any right whatsoever to decide who you must list for a default. And I am quite confident that players need not show up for a match that has been defaulted in advance. http://tt.tennis-warehouse.com/showt...ight=grievance
Now, say the situation were a little different. Say opposing captain did not give advance notice of the default and instead handed you a line-up cart with "DF" on it. Your line-up would already have your two players written on it.
At that point, I do not believe you would be allowed to change your line-up and your two players would need to be there to accept the DF. You could always ask the opposing captain to let you change it, but if she objects you are stuck.
Bottom line: I am siding with you, OP, unless your league rules are different from ours.
[edit: I should mention that I think you should not accept opposing captains default if you don't have two available players. In that case, it should be a double-default. But if you have players A and B available, it is OK to take the default and list players C and D as receiving the default, even if C and D were not available.