In order for the singer to hit the high notes on the Hollies hit song 'Long Cool Woman in a Black Dress' I’m considering detuning my entire guitar down one whole step, in other words, bottom E string to a D, high E string to a D etc. etc. How does this fit with the experience of this group to accommodate a singer for the song? Also this tuning could accommodate the singer to hit the high notes in Rock ‘n’ Roll by Led Zeppelin.
I'd normally say 'learn the chords in the lower key!' But that particular song does have a characteristic riff that uses the bottom string, and tuning down would be a reasonable option.
Whether you'd want to do the whole gig on a detuned guitar is another matter. Have a second instrument set up, or accept the hiatus while you re-tune.
It's one way to do it. Especially if you need the guitar to play open E or A chords or strings. Like guitars often do in rock songs.
The problem is then tuning down/up between songs, or leaving the guitar low tuned. Which means other songs that need the open strings/chords don't get them. If you keep your guitar tuned down a step, you could capo on 2nd fret for the other songs, although it's not the best way. Or, change the chords for the rest of the stuff you do! A lot of effort for little gain, although the singer may or may not appreciate it. Or, merely change the key and use the appropriate guitar parts in lower keys. A barred D chord isn't going to sound much different in context than a barred C (to most people...).
There is now a magic box on the market which changes key for you, with no perceivable delay, so you can play whatever you like, and tell it what key it needs to produce.
Another problem that I used to come across, especially with Hollies and other vocal harmony numbers, back in the '60s, was that when the key of something was changed to accommodate the highest voice, the others, particularly the lowest, would sometimes suffer, as that harmony became too low.