It's possible, but there will be some problems.
You need to rebind keys in your ~/.mplayer/input.conf (see example in /etc/mplayer/input.conf or something like that, depending on distribution). If you'd like to disable all the default keys, use command-line option -input nodefault-bindings.
As for what you need to change, look at http://www.mplayerhq.hu/DOCS/tech/slave.txt
. But remember that "pausing_keep " would still play through some frames while the command is executed, and then pause again. Also, this frequent pausing/unpausing will break audio/video sync temporarily (it will be gradually restored, see command-line -autosync option). There's also "pausing_keep_force " which doesn't pause/unpause but is experimental and not guaranteed to work.
As for me, I was particularily bothered by the fact that video starts playing when I just want to see time elapsed (key P), so I used the following line, which toggles pause and displays time on Space, and was done with it.
SPACE pausing_toggle osd_show_progression