Apology, but...
I do apologize if my original comments were taken as a personal attack. I certainly did not mean them as a personal attack, though, I do stand by my comments that the "paintball hunting" idea is not wise. Allow me to elaborate in a hopefully more polite manner:
As you can see from these first replies, other hunters would not appreciate this action. Hunters tend to be very concerned about the safety and quality of their game as well as the actions of other hunters.
For example, if someone in an area starts to see deer wander around with red and yellow splotches on their hides, he's going to think it is very odd, and wonder what's going on. He may take the game, then he'll have to register it at a local station. The station, also thinking it's odd, will report it to the DNR office in your area and they will investigate. Thus, they're on the trail right away.
And, if either 1) it happens more than once or 2) anyone is told about it, then they might well track down the perpetrator and,as mentioned before, inflict serious penalties.
With this in mind, it would be legally unwise to do so.
In these days of waning support for hunting and gun ownership, one of the last things the effort needs is local press about what many would consider to be irresponsible hunting.
It has been my experience that many non-hunters can swallow the idea of killing game quickly and cleanly so they do not starve and overpopulate, but most cannot accept shooting them merely for the sake of doing so.
With this in mind, it would be politically unwise to do so.
Personally, I go hunting quite a lot, but often, I come upon fine game, but simply decide not to shoot because I don't really need the meat for any real reason. These hunts end up being very slow, pleasent walks through the woods.
Realizing that I'd like some record of my stalking ability, I am now in the market for a good camera. Consider "camera hunting" with the two points above. It is very legal, AND it is very politically safe. (afterall, consider how many "greens" are bird watchers, etc).
As for crosshairs in the lens, this sound like a pretty good idea. I don't think you'd have to get them custom-installed, though: I've seen attatchments for some spotting scopes which allow them to be used as the lens for a SLR camera, and some of those spotting scopes have cross hairs.
Best of all, you have a permanent record of the hunt, AND you can do this year-round (i.e., not freeze your keester off in late november).
Anyway, those are my thoughts. I hope I got the message across in a more civil, less personal, way.
Happy (camera) Hunting!