The attached file implements this support. Note that the score="distance" local-param feature in the Solr 4 spatial field should also work too – pretty cool.
I took a look at adding support for the "geodist" function, which is implemented in HaversineConstFunction. Ugh, the logic therein is convoluted and it makes heavy assumptions about a "MultiValueSource" based field which the Solr 4 spatial fields don't use and never will. Adding support here for Solr 4 spatial fields would definitely be a hack and on top of confusing code that I don't want to make any more confusing. So I'll pass on that.
I plan to commit in a few days.