Full acceleration means that the transmission is operating with sufficient force to prevent the disengagement of the gears. It's more likely that such an attempt will break something which was not designed to accept the force necessary to make this gear change.
If you consider the forces applied to the gear teeth of the transmission under acceleration, there is friction (despite lubrication) preventing the sideways motion necessary to disengage those teeth. The detents of the system provide similar but much smaller levels of force.
I have shifted manual transmissions multiple times by relaxing the throttle, selecting neutral and re-engaging when the engine speed matched the desired gear selection.
One can apply pressure to the shift lever and it will not disengage until or unless the load is released.
Due to the internal forces of a transmission, it's not impossible to consider that one could begin to move the shift lever sufficiently to reduce the contact between gears, at which point the gears would fail mechanically, followed by a good portion of the remaining transmission.