Can some device record and play in the same time?
Not in rockbox, although I suppose if you were motivated enough you could write software to do that.
Edit: I suppose if you just want to hack something together, theres no reason you couldn't just ignore our playback/recording code and have a plugin directly control the audio hardware. Taking the line in data, buffering it for 100ms and then playing it back through the dac should be pretty straightforward.