a. narrowness
I limited this to coverages and elevation because it was what I needed, and I did not want to expand it to areas I do not understand and break something.  I had two CoverageInfo types I found to test for elevations:  1. in a past version of GS, coverages based on the Geocent NetCDF plugin. 2. in that past and in the current version of GS, coverage based on example ImageMosaic layer described in the GS tutorial.  I am not aware of whether this applies to FeatureType layers and if so, how to set up a test layer of that type.

b. dropdown with tens of thousands of values
I had not anticipated this.  I don't want to send tens of thousands of values across the wire and have the client's browser loading the dropdown with them.
Ideas I can think of:
  - use text box instead of select.
  - use select conditionally if number of options is less than some number.
What do you think?