You'd run the risk of your ex's jealousy, even if "it's over". You'd talk with other gals, she'd be a negative wing, saying this and that bad thing about whom you talk with, she might want to leave early..etc. That's the risk.
What if your ex called you up, and wanted to use you as social proof at a party, etc? You walk around, while she is really trying to attract other males. You'd be tempted to c*ck block.
However, Mystery does advocate the use of "pawns". I've hung out with gals that are just friends, and were never more than that. It really does make conversing with other women easier. I get looked at, then I open and their IL is higher.
My personal advice is to not misuse anyone, especially an ex.. she and you shared for a time. If she's up for going to an event, and it's clear you are both singles, you may want to try. But, treat the first outing as a learning experience, don't expect it to work.