Method for Geolocating Datasets

Howdy all,
I think we're making some serious progress here, very exciting. We have a search map displaying on the search.html template and the solr schema is updated. Next stop: defining the geo extent of each database.
The Docs are uncharacteristically thin on this point. I believe this is all we get:

> Regardless of the backend that you are using, in order to make a dataset queryable by location, an special extra must be defined, with its key named ‘spatial’. The value must be a valid GeoJSON geometry, for example:

So I am assuming that "special extra" means "custom field"?
Ie, we are just going to use the standard input form, and take over one of the custom fields?
I will try this now, but can I ask if anyone has coded out a solution to draw a bbox or point on a map to derive this input?
Thanks!
JB