Something like that. Except I just started using customline. The part we're concerned with looks like this:
# This is the part of the WPS used if Album Art is present.
# The %?C tag comes AFTER the customline -- VERY IMPORTANT!
%e|16|130|124|1|FFFFFF|%?C<%s%xda%?it<%it|Unknown Title>|>
%e|16|143|124|1|FFFFFF|%?C<%s%xdb%?ia<%ia|Unknown Artist>|>
%e|16|156|124|1|FFFFFF|%?C<%s%xdc%?id<%id|Unknown Album>|>
%?C<%pb|6|2|140|172||>
%e|2|181|138|1|FFFFFF|%?C<%al%pc%ar%pr|>
# This is the part of the WPS used if Album Art is NOT present.
%e|16|78|124|1|FFFFFF|%?C<|%s%xdX%?it<%it|Unknown Title>>
%e|16|91|124|1|FFFFFF|%?C<|%s%xdY%?ia<%ia|Unknown Artist>>
%e|16|104|124|1|FFFFFF|%?C<|%s%xdZ%?id<%id|Unknown Album>>
%?C<|%pb|6|2|140|120|>
%e|2|129|138|1|FFFFFF|%?C<|%al%pc%ar%pr>
Basically what I do is make all the customlines, and use conditionals to determine whether anything will actually be displayed on them or not.
%e doesn't work in conditionals, and it's ugly if I code it that way anyways.