The 78 supports Custom 'raster' images so you could also view USGS 24k (or 100k) quads.
Unless Garmin has increased the number of cells, you will be limited to about 1 quad at a time.
Additional map data is usually added by creating an overlay/transparent mapset. Unfortunately, polygons in such a mapset will display after and cover up the data in the 'primary' mapset. Not too useful as T&R data is esentially contiguous.
The polygons could be converted to lines; however, each common boundary will have two lines and it may not be too clear which T&R (section, etc.) the label is refering to.
I have not used custom types; however, I would expect you could create some polygon type that would be 'invisible' or minimally visible - somewhat like what jbensman has done in his land ownership maps -
http://www.gpsfiledepot.com/maps/view/247/Something on my to-do list.