Yup, just replace that D ring, or see if you can rig up something using a paper clip. That'll let you adjust tension while riding.

Keep in mind that friction mode with indexed shifters may not work well. Depends on the design. My Suntour GPX downtube shifter modules have two index modes and one friction. The friction mode doesn't have any clickstops and relies solely on friction between the washers. The problem is finding a balance between good friction feel and retention of the desired position. It tends to creep and ghost shift unless I really crank down the pressure -- in which case it loses any reasonable tactile feedback and I end up missing shifts.

It wasn't too bad in friction mode with an old Shimano 600 rear derailleur, but still tended to creep and ghost shift when standing to climb or sprint. I tried it that way for months and finally got tired of it. Switched back to the original Suntour GPX RD and index mode last week.

A purpose-designed friction shifter may work better. My other shifters on various hybrids, including a dirt cheap SunRace thumbie set and a Shimano bar-end shifter with index/friction modes, both include clickstops for a better feel and better resistance to creeping and ghost shifting.