I don't think this is possible hence the feature request. I suspect I can work out a way to do the specific things I am thinking of without this but it feels like a useful feature to me so raising it anyway.
I realise a smartlist is dynamic, i.e. it is a saved search (so the results update as the library does). I would like to do is to be able to extend this with user input, i.e. a parameterised smartlist.
for example I could have a search like
[Duration]=<=100m [Media Sub Type]=Movie [Media Type]=Video [Last Played]=[] ~sort=[Date Imported]-d
and I could define a view for this and get a nice list and be able to further group that and so on
what I'd like is to be able to define that something this instead
[Duration]=<=? [Media Sub Type]=Movie [Media Type]=Video [Last Played]=[] ~sort=[Date Imported]-d
i.e. mark 1 or more predicates as parameterised
I would then like the view to present an input field for each parameter to allow the user to define the actual search results.