BuddyPress.org

>Is there a way to pre-filter these before the ‘<l1>…‘ is setup?

afaik, no.

The filter hook is in a foreach loop.
So you for each item you get an array that has 2 elements- the li code as a string and $user_nav_item.

So your function could do a string replace on the li item and then echo the result.
Or use $user_nav_item, except for ‘name’ and create your own li item and then echo the result.