Well actually....
Even if you place the exit at higher altitude with lower pressure, without a seal you'll still end up with higher pressure at the base. Just like you do in open air, the weight of the air column pushing down increases the pressure.
Even if you place the exit at higher altitude with lower pressure, without a seal you'll still end up with higher pressure at the base. Just like you do in open air, the weight of the air column pushing down increases the pressure.
If the launch vehicle can accelerate well enough in low pressure instead of vacuum then maybe you could "just in time" depressurize segments of the tube. Although that's still getting perilously close to the magical airlock concept.
Apologies for rambling, this is fascinating.
A many kilometers long linear accelerator that ramps off a suitable mountain could make for a interesting 1st stage that wouldn't subject the payload to ridiculous G forces. But you'd still need to accelerate the payload and 2nd stage and with SpaceX and co driving down 1st stage costs I still don't see it as cost effective.