There's a special sense of humor in maintaining a panel with "longitudinal" in the name but making it impossible for users to download data from more than one time point at once.
LISS has a basket option which automatically merges the time points/datasets you need. Is this what you mean?
(also the "time-invariant" background variables tend to be simple copies, except when they are actually repeated; so a single copy is often enough, depending on the time period)
I have just been informed that the (time-invariant) background information is somehow spread out over 200 data sets, each of which has to be downloaded individually.