Another code question: can I take a 110V outlet off one leg of a 220V circuit?
There are a few concerns:
Neutral and Grounding
First and foremost to do any of this would require a neutral conductor and a non current carrying conductor known as a ground/earth.
Undersized Grounding
Most double pole branch circuit grounds are only sized at #10. This is only good for up to 60 Amps according to the NEC.  The total Amps gained with the 120 V outlets may exceed what the code allows for #10 grounding. If the existing double pole is rated at 50 Amps, than you have 10 Amps to spare before having to increase the size of the ground to a #8  If you plan on removing the double pole receptacle all together than there shouldn't be a problem with a undersized ground.
OverCurrent Protection
If the above criteria has been met and the existing double pole receptacle is protected by a double pole 15 or 20 Amp breaker then you can safely tap off the conductors at the junction box to fed other 120 V outlets.
However if the double pole receptacle is protected by a breaker greater than 20 Amps, then one solution would be to change the the double pole receptacle to a small main-lug sub-panel.  Then branch off the sub-panel so each branch circuit is protected by its own breaker.