Hi Andrew. There seems to be a lot of "inspect" in some of the service schedules I've seen. In others, they recommend transmission and differential changes every 30,000 kilometres or 2 years; if that were the case I'd be changing both every 11 months! A USA schedule recommends these short intervals if used for towing, but otherwise doesn't seem to mention it. So, yes I'm confused as well! When my old girl went to Lexus (ouch) at 100,000 miles I asked for a transmission change and they said "...it had probably never been done..." before they had even got it on the ramps, so I guess as you suspect with yours, it's not a routine item other than "inspect". Seems like a good time to do it.
At 150k, I got my local friendly mechanic to change it again, and the differential oil at the same time. Both are quite easy DIY jobs but nowadays I prefer him to do it for me(for many reasons). I think I'm right in saying all IS300 sold in the UK have an limited slip differential, so need a differential oil with an LSD additive. 100k again is probably a good time to do it, should see you right for the next 100k!
Don't know about Terraclean, but I think that is more beneficial to direct injection engines isn't it? As for the blanking plate, I will have to take the lid off mine and have a look. There is a video on YouTube of a bloke removing the throttle body on an IS and cleaning it. Lots of pipes to keep track of if you do. While you have it off the car, may be opportune time to change the plugs as well, as they are recommended for replacement at 100k.
Do you have the workshop manual for the IS?