Well, of course you can tune a guitar down. Like you said, there is a cost in tone (and tunability -- too-slack strings don't fret the same as those at tension), but it can be done. I'd recommend not going more than a whole tone, though, to minimize the "cost" as mentioned. Some guitarists who play with reed sections of bands tune down one fret's worth, so that Ab, Eb, etc., are easier to accomplish than usually.

The main problem I see is simply keeping "up" with other players, since, just like when you use a capo, you have to think at odds with what your fingers seem to be doing. Watching the other guitarist(s) becomes harder when you aren't in the same tuning (whether it's "standard" or not).

And the feel of the strings (tension, etc.) is different, too. You can fret too hard if the strings are loose, and hard picking gives you odd "effects," and not necessarily the ones you want.

But you can do it. Let us know how it works.

Bob Clayton