You actually have some choices, but they will force you to wear the engineering hat. One of the things you can do is buy a flashlight that has a squeeze button. The squeeze button can be lightly pressed to temporarily turn the light off (being on) or to turn the light on (being off). Yes, it's clumsy and really an awful workaround than anything else but know that that's an option.
If you tinker with it a bit, you can create an extended trigger that will be placed near the shutter release button so that the minute you press the shutter release button fully this light will turn off (its only purpose is to help assist manual focus). You are not going to believe this, but you might be buying an aftermarket trigger meant to toggle flashlights typically attached to tactical assault weapons. See www.dealextreme.com (they ship slowly, but surely, and decent customer service) for such flash lights and triggers. Being a photographer means good bed-side manners, so don't take a LED flashlight and blind your subjects, think about ways to considerably soften the light while still lighting the situation sufficiently for you to focus.
This is a huge bummer of course, so what I do is simply to ask the venue to please turn on the light a little bit, which works out pretty well when doing events photography.
Another thing you can do is to get really good at flash photography. Once you are comfortable with flash photography you'll be able to kiss the ambient lighting condition by a stop or two, that of course doesn't help you manual focus, but it does afford you the chance to stop down the aperture. Once you've said goodbye to f/1.2~2.8 and hello to f/4~f/8 you'll get a generally deep depth of field--less focusing issues then. From then on you just do zone-focusing by manipulating the focus ring of the lens to the distance marks ticked on the barrel.
If you know you'll have some ambient light, then your last choice is to swap out the default DSLR focus screens for after market focus screens condusive to manual focusing. You'll still need a little ambient light, swapping out the focus screen won't help you focus in pitch black.
Lastly, get great professional lenses that can continuously auto-focus with the help of a powerful focus assist lamp from an off-camera flash. That's really the thing to do especially if your subjects are a moving target. All too often they ARE moving targets.
If this is already old news to you I hope it will help others.
Best personal regards,
