I agree, just don't do it unless you're absolutely sure, with a 100% certainty that the other party is alone.
So what story do you have? Indeed it would be tough to come up with a generic excuse for each particular situation as some things may come up in the conversation that would be hard to dance around. In some cases, you may be able to pull off that the SP, MP or client is mistaken or crazy (hopefully names don't come up), but then if names are spoken you're pretty much SOL. You'll have to come up with a common place you may have met which is believable for both parties.
For me, a believable story would be that the 'previous meeting' was at a bar socializing after work with co-workers. Before any kind of conversation gets started, alerting each other that an SO is nearby would be the best thing.
As mentioned earlier, just don't approach or initiate contact.