OK, so this is a crazy question in many ways but when working for NHS and banks you come to realise that sometimes these old browsers just have to be supported.
I remember ctools did a lot of work to support old IE versions (i.e. CGG!) specifically for these use cases.
However; Someone seems to have forgotten that when implemtning requireJS.
In particular, this file is the culprit, it makes several uses of forEach syntax, which is not implemented in IE8.
Now, I googled around require and I don't see any reason why it wont work in IE8 - so it's just this (and possibly other) Pentaho specific files which are causing the issue.
Does anyone think it should work? Did anyone try? Are there other blockers further down the line?
Just to back up my story, the UK government does still recommend that IE8, 9 and 10 work, but they don't have to be fully functional, and are indeed likely to have display issues. See here:
By the way, forEach was implemented in IE9, and these requireJS dashboards do load fine, so this particular issue does seem like a relatively easy win?