A quick example of how to use a universe register to remove a wall from a game using a platform trigger , you can use any free UReg number you choose I just used 8 for this example .

Use the wall image you want to remove for each frame .

frame0 Skip if UReg=value ureg#8 uvalue 1
frame1 Goto frame 0
frame2 Delete myself

Now lay a platform and select "Platform sets a Universe Register" , put 8 in the Universe register Number box and 1 in the Number to place in this register box

That's it , when you step on the platform the wall will disappear , you can use this method to remove any objects including MD2's .
You can also put objects into levels by having frames 0 and 1 have no objects in them then have 2 more frames that loop to each other that have the wall/object defined in them .