I modelled a movement tray for my TG unit to be a flight of stairs down from a temple, when I had this argument with a friend we agreed to a house rule eliminating the idiotic TLOS rule when I reversed the tray so the TG were climbing up the stairs and you couldn't see any of the slann.
We also agreed that the fluff saying that the slann raises up to gain LOS would in any case overrule TLOS as army book trumps BRB and specific trumps general. I.e. the specific rule the the slann has to rise up out of the unit is a specific rule in an army book and thus overrules the general TLOS rules in the BRB.
We are also extremely biased against TLOS unless you have terrain specifically built to work with it, i.e. hills high and steep enough to affect TLOS, woods that actually block TLOS through them (the edges of real woods block LOS, while LOS within in mature ancient woods can be relatively unobstructed at ground level).