The problem with quicksand is that it is normally indistinguishable from any other patch of sand, so if you wanted to put in quicksand please do so very sparingly.
Here is my idea for adapting it to Mecha.
A patch of quicksand with at least 5 tiles adjacent to each other are generated, If a walking, rolling or otherwise on the ground Mecha, the mech in question has to pass a movement speed roll (Think like a resistance check, only with a much higher target and a much higher baseline) first (I.E going too fast to notice it), After that the mech in question has to pass a resistance roll every playerturn or else suffer a 50 speed penalty, if this keeps up then when the mech stops moving alltogether then its pretty much grounded in -2 waters, so unless it can make a good engine check (using the mech engine) then its stuck.
Yes it sounds a bit complex, but I think it would work.