I don't own either thing yet, but if I get a stegadon model and make it the EotG, do I (or can I) use a skink priest on foot anywhere on the thing so that he can be removed and used in lower point games (or when the steg dies and he survives?) I'm going to try to build up points for an escalation type thing, and EotG just doesn't fit in 500 points.
Fer is right, there are no bases in the Steg kit, but assuming you have some spare bases kicking around (and really who doesn't?) it shouldn't be too much trouble to either pin him (without glue) or use a small magnet to hold him onto the Steg and also make a base he can be placed on.
Since he is a character and chances are at one point or another, you are going to find yourself with the steg killed and the priest still living or vice versa. You can either use a second model on foot to represent him (grab a priest from another kit) or just put him on a base and put him on the steg. The steg howdah is a bit ramped when you have the throne on it, but there should be a place for him, and yeah he could go anywhere on the howdah. It won't affect his LoS since he would use the steg's anyway.