well, I figured this out myself. for the benefit of others who come upon this search:
using corridor properties, surfaces and boundaries tabs create a corridor surface using data type "feature lines", adding crown and ETW; then use ETW to make automatic boundaries; add this new surface to the SLG so it will be available to you; then in section view group properties, toggle on this new surface and change the labels column to use a label set w/ a segment label style for % grade; since the new top surface only has segments from crown to ETW, you'll only get one set of slope labels