I was going to recommend a Hobby charger but seems someone beat me to the punch.
Regardless, you're better off charging all the cells separately but like you I understand how that is kind of a PITA. It's one of the reasons I always use standard battery packs rather than cell holders.
The thing is since you're using a cell holder it's just better to charge the cells separately. The metal contacts on cell holders will accumulate oxidized residue over time. Believe me it doesn't take long for this to happen and it isn't visible to the eye. If not cleaned regularly the residue builds up and small voltage drops begin to take place. While this isn't really that big an issue it does exist. More a problem with single cell applications than with cell holders. Still, NiMH cells are just 0.3 volts less than standard Lithium ( non-rechargeable ) AA's at 1.5 volts so the difference in a couple tenths of a volt will shorten the run time even if only for a couple of minutes. Even more an issue in colder temps if the lamp you use has automatic power-down circuits ( which the 200L does ).

I have one of those old 200L's. I used to use the same set-up as you do only back then they didn't sell the Low self-discharge NiMH cells. Eventually I bought a Li-ion ( 4 cell ) battery with a 6 volt regulator built into the pack and that worked great for the few times I used it. The Li-ion pack had almost twice the run time of the NiMH's but the downside was that the pack would self discharge in storage because the regulator circuit was always, "on". That meant I had to make sure I charged it right before every ride. The 200L , while a fine lamp is just obsolete now. If you're going to spend money you might just consider a better/brighter lamp. Besides if you buy a lamp that uses Li-ion cells the Li-ion cell just have larger capacity and handle colder temperatures better.