Thanks David, I’m loathe to use character styles to force alts because you can only apply one character style at a time (I might want to put some of the text containing an alt character in italic, bold etc.)
It turns out there’s a pretty straightforward way!
I’ve done some digging around and (I think) made some sense of stylistic sets. These are available to apply in the Opentype tab on the paragraph style dialogue box, you just have to know what stylistic set does what (and that’s the main problem with this). InDesign names these sets numerically (you can’t change them), so the key is to check the font documentation, it should give you a list of what set does what, just note the number and select it in InDesign. I don’t know why ID/Opentype don’t seem to support custom naming of these because it’s a great feature, just not particularly intuitive to use.
Obviously, this only applies to Opentype fonts with alt features.
Anyway – I hope this helps someone with the same issue…