I am trying to map Alt+Space to Escape in Puppy Linux. Specifically, this is for vim, but I could not get it to work there. So, I thought I better try to do it globally, but I have no idea what is the key code for Alt+Space.
You can't: xmodmap can manipulate modifier-keys and manipulate key-codes, but Alt is not one of the predefined modifiers (i.e., Shift, Lock, Control).
Shift and Mode_switch are treated specially, and allow the sort of manipulation which you have in mind.
You can assign Alt to one of the placeholders
mod5. That lets programs see Alt as a modifier, but does not provide the the ability to treat it as one of the shiftable-features.