I've been soaped as a kid but never while spanked. Personally I wouldn't try it. Hard spanking makes you draw a lot of air into the lungs, and soaping causes copious foamy drool. So this seems like the perfect recipe for a lot of aspirated soap (and, worst-case scenario, aspiration pneumonia).
What could work more safely is coating something solid that's too large to swallow accidentally with a bitter or sour taste that doesn't cause foam. You could soak something porous in lemon juice just to try it out, then graduate to nastier but still safe stuff like Mavala Stop. (That's the stuff some parents use to deter thumb-sucking. I've never tried it but it's supposed to be horribly bitter.)
If you'll indulge me getting a bit creative, I also like the idea of being made to eat or drink something highly salted or sour prior to spanking, so that the taste remains in the mouth but doesn't pose a choke hazard. Your girlfriend could prepare a "punishment dinner" with your least favorite vegetables coated in salt or hot sauce, with straight lemon juice or bitter tea to wash it down. There could be interesting penalties for not cleaning your plate, or for not finishing within a certain amount of time. If you were gagged immediately after and spanked, you'd still have the horrible tastes in your mouth but no choking hazards.
Whatever you try, I hope it goes well and you'll let us know about it.
