Nope, nope, don't do it - dangerous situation for several reasons. The rule is never launch the Mavic without an excellent GPS lock.
Why? Let's suppose you do. The home point that would be set after takeoff, may be based on an unstable GPS signal reflected from the canyon walls. You can't verify the location using your map home point, because your phone/tablet GPS is no more accurate than the Mavic's. If you have satellite maps, you may be able to visually verify that your home location is correct, or set it accurately in Litchi, but more than likely you don't have a good GPS lock and/or satellite map display on your phone or tablet either.
So you're stuck. If you can't get a good GPS lock on takeoff, then you should probably disable RTH because you don't know for sure that the home point is accurate - it could be a kilometer away. If the Mavic reaches sufficient altitude that you have a good GPS lock, and the Mavic is over a nice large landing area, then at that point you could safely set the home point to the current drone location.
In addition to compromised RTH functionality, which is bad enough, taking off with an unstable GPS lock risks a fly-away each time the GPS lock changes, potentially altering the Mavic's apparent position. Each time this happens, the drone will go flying off to get "back" to it's previous lat/lon location, and there is really no limit to how far away this can be.
But flying down a narrow canyon is not a good move, even if you start with a good GPS lock and a good RTH position. If you fly down the canyon, you risk losing GPS and going into ATTI mode! Nasty enough, but you are also at increased risk of losing rc signal and/or video. If, on the other hand, you fly up and over the edge of the canyon you risk losing your remote connection, and you are now completely dependent on RTH, which may be very inaccurate.