Shadowseer and transport tactical situations

Hey there. I'm having a bit of a tactical dilemma.

I can't find anything in the rules that allows you to cast spells from within a transport even if it is open topped. So if I put my shadowseers into transports they become useless effectively until disembarked or the transport is destroyed.

How best then to utilise them? I am planning two shadowseers in a 2000 pts list but I'm wondering if they are worth it when all my troupes will be in transports or is it only worth taking one?

Or should I assume turn 2 will be my Hail Mary turn. I personally don't want to put my clowns out in the open hoping to cast the movement spell on them because if that fails then they are bullet fodder.

Also I think the harlequins work well when moving together. Moving a single unit in isolation is not ideal. The other alternative is to hide the shadowseers behind the transports so they can be functional from the start of the game, but this is risky. I like the idea of taking them because theyre a source of mortal wounds and buffs.

What experience have people had with using shadowseers in 8th?