In R, how can I build a list with some elements referring to earlier elements in the list?

You can use dplyr::lst which is same as list but here you can build the components sequentially.

dplyr::lst(
  width = 128, 
  overlap=width/2,
)

#$width
#[1] 128

#$overlap
#[1] 64

Another option is:

opts = list(
  width={width<-128},
  overlap=width/2
)