Ok, this might work but it may not travel well. Can you assemble your palm tree on site?
If so, here goes.
Okay, ahead of time (so there are ample days for drying) take gumpaste and form your trunk. If you want to beef up the internal structure, use some uncooked spegetti sticks (or is that not allowed?).
Then, on top of your truck, form a ball so you have something to attach your leaves to (make the ball the same color as your leaves so it blends in between them).
Also, with gum paste, roll thin leaves and make little cuts along the sides at an angle with an exacto knife. Let these leaves dry on something that give them a slight curve so they will hang downward when attached to the ball portion.
Then, when you get to your venue, set up your cake, take some melted chocolate, put a blob where you want to attach the trunk to your cake board and put it on. (Also, if you are using brown sugar for "sand" you could sort of pack it around the base to cover any excess chocolate and lend extra support).
Let it dry a little and then use either royal icing or melted chocolate again to attach your leaves to the ball and let dry.
You may want to bring a can of air too so you can spray it as you attach things. Just be careful not to blow off pieces already attached LOL.
I'm not sure how big this thing has to be, so hopefully you can make something work.
Also, just FYI on the palm leaves, if they don't need to be too big, you can use a holly leaf cutter, cut them out, roll them a little thinner yet to stretch them into a longer leaf and then make the slits. See my Dora cake for examples, that's how I made those leaves. Best of luck. Post pics!