Someone recently on this list was saying something about ways to embed 
facets in for instance Atom feeds.

I was reminded of that, because checking out an Atom feed from Google 
Books Data API, in Internet Explorer... Internet Explorer displays 
'facet' type restrictions for it, under a heading "Filter by category".  
It also displays sort options, apparently somehow the feed is 
advertising it's sort options too in a way that a client like IE can act 
upon?

Haven't looked into the details, but here's an example feed: 
http://books.google.com/books/feeds/volumes?q=LCCN07037314

Look at it in IE for instance.

So whatever's being done here is apparently already somewhat standard, 
at least IE recognizes what Google does? I'd encourage SRU or whoever to 
follow their lead.

[I agree that simply copying the Solr API for a standard like SRU is not 
the way to go -- Solr is an application that supports various low-level 
things that are not appropriate in that level of detail for a standard 
like SRU or what have you, at least not until they've been shown to be 
needed.]