Lots of good suggestions here which I'll look into more and think about, thanks!
The cases that Rug likes are the sort I have and are seeing models chip/get damaged in (maybe I just don't pack enough extra foam in to each rectangular hole with the model(s) in it. The sort that The Real Chris and Man of Kent have recommended look better, but I still wouldn't trust resin Tau and Grey Knight models in them. Then if I'm going to aim to make some kind of magnetic case system for those models then if it works I may well try to adopt it for all models.
Transport aside I'm worried about how the Tau and Grey Knights models will survive game play handling, one poor Kroot's feet fell off when I picked him up (and another couple were broken in their bags). I know some people glue banner poles to their Tau bases and pick them up with that, but personally I'm not a fan.
I'm musing on two different possibilities at the moment:
a). model my Tau as fighting in the same jungle terrain as my Exodites will be - have lots of decoration and jungle trees growing up on every base and pick them up by the trees. Like below, particularly the third one on the right (foliage will be added later). This would be particularly fitting for Kroot too.

b). when on display in my cabinet or to photograph etc I don't want anything sticking up that shouldn't be there, but gaming it could be useful. Since I plan to use 25mm round BoFA bases I have with some thin metal underneath, what if I epoxy glue small magnets onto the ends of pieces of cylindrical clear plastic rod (say 2-4ish mm diameter) specifically leave a flat-ish area in the middle or somewhere on the base and then magnetise it there when gaming so that the clear plastic rod sticks up and the base can be picked up by this (sort of the shape of a flying stand shape). Could this work? Only downside I can see is they would have difficulties going underneath thing as the sticky up rod could get in the way e.g. a Manta or overhanging terrain, but that shouldn't happen very often and they could always be detached if needs be. I would need to find magnets strong enough to pick them up with, but weak enough not to get overly excited about magnets on neighbouring 25mm bases or other players metal watches brushing past, etc.
Thoughts?