Hi,
You could probably abuse group sorting for that, for example:
autocmd DirEnter / setl sort=-groups,dir,name sortgroups='^(tmp)$'
This moves /tmp directory to the top of the list because it's the only one with non-empty group match. Multiple directories can be listed in 'sortgroups' regexp as well.
It has implications for things like :sort dialog and overwriting current sorting which might or not be relevant for your use cases. Something like let &l:sort = '-groups,dir,' . &g:sort should address part of it:
autocmd DirEnter / let &l:sort = '-groups,dir,' . &g:sort | setl sortgroups='^(tmp)$'