This can be done, and I have done it.
To begin find the low cost "laser pointer" found mine at the dollar type store. This has three button type batteries. Take the bulb assembly out of the focus spot. And now go to the hobby store and fine the brass tube that slips one size inside another (K&N is the brand I use) and find the size that fits inside the focus spot, and the necessary smaller sizes to fit over the purchased pointer. What you are doing is making the sleeve or sleeves to keep the rangefinder centered in the focus spot housing. In my case I made the outboard battery housing and switch assembly and wire leads that go to the range finder. With the left hand I can turn the light off and on while holding the camera and focus with my right hands. This has worked for me.
Since I do not do digital will have to ask a friend to take the photos of what I have done if anyone is interested.
Where the focus spot attaches to the rangefinder there may be the small lens, depending on pointer and such you may want to remove this lens. A very simple way to do this is remove the focus spot and just aim laser from the top of the rangefinder.
A reminder do not look into rangefinder while doing this. You will have the two laser spots going forward and one laser spot going to the rear from the eye piece. Do not look at laser!

Good luck