Yes you can. Somewhere (i am not quite sure where) i saw a 30g co2 regulator. It takes the pressure down to 1-10bar (not adjustable but you can by 1 bar 2 bar etc.) Considering to make an antweight move you only really need about 20g i think fitting in a 30g reg would not be hard.

Chris Cummins found some Co2 capsuals for Sodastream that were disposible and lightweight and cheep. (Roughly 5quid for 10). These would be perfect. If the pressure was regulated down to about 5bar then a lego valve can be used. And if a lego valve is used then so can a lego ram.

The whole set up would probably weigh no more than 70-100g leaving plenty of weight to make the robot move.

Regards
Ian Mc Donald