Well, of course you wouldn't want to moisten the inside of an acoustic - that's absurd. The sides need to retain their shape and the wood needs to maintain its integrity. A fretboard is a different story. You're not asking a fretboard to contort, work with bracing and fight tension the same way the back and sides of an acoustic guitar need to.
I've been using lemon oil to condition rosewood fretboards since the early 80s, on an irregular basis. I haven't had any problems one way or the other - no fretboards have dried out & cracked due to nothing being applied, no fretboards have unseated frets or separated from the neck, or suffered any ill effects due to having been somehow "over-hydrated" by one occasional application of lemon oil. I see it as no different than applying lemon oil to your woodwork or furniture once in a while or wax to your car. It looks nice. If I lived in a desert climate, I suppose it's possible that it might help prevent cracking, like applying lotion to your hands on a dry winter day.
It's very important to finish both sides of a piece of wood to maintain stability. You want both sides of a board to absorb or release moisture at the same rate. Otherwise one side expands and contracts faster than the other and the wood warps. This is woodworking 101. So why are the insides of hollow bodied guitars not finished? Because a) it would be too hard to do, b) the various parts of a guitar, including the bracing, stabilize each other, and c) the wood is thin enough to prevent moisture content gradients. Nonetheless, sometimes hollow bodied guitars fall apart, and moisture instability is one of the reasons. That's why the owners of fine acoustic guitars try to maintain them in a stable humidity.
If you did manage to oil the inside of your guitar it would have no adverse effect on its integrity.
Oil will do nothing to help hydrate wood in a desert climate. Hand lotion is mainly water, not oil.
Oiling your woodwork and furniture is not the same as oiling an unfinished fretboard. Unless you have handmade oil finished furniture (I do) your furniture and woodwork has a film finish. The polish doesn't go into the wood. All it does is make the furniture shiny for a brief time. Oil is a reasonable finish for furniture that isn't going to get hard use, but it has to be renewed periodically --mine is way overdue--and it will wear off quickly if the piece is handled a lot, like a guitar neck or fretboard.