Since you mention this I was wondering if it would be possible to add an option ("layered navigation display type") in the backend under “manage attributes” > “frontend properties”
It would allow us to choose between dropdown/checkboxes (or the default list) for displaying layered navigation options in the frontend. This counters clutter when using lots of filterable attributes and would allow us to make an attribute ‘multiselectable’ for layered navigation even though the backend thinks differently.
I’ve added a mockup to show you what I mean.
Click thumbnail to see full-size image