Is there any way to write a single line of music, assign chords to it, and then have Dorico fill in the chords WITHOUT changing the top voice, but rather using the top voice as the LEAD voice in a tutti soli for example.
How did you wish to see the chords realized? Into a grand staff? If so you would need to add a grand staff and then select it as the recipient for the realized chords.
One does not have to print the layout with the grand staff in it unless one wants to; one can just print the solo line with chords layout.