The thing to do is start them out with a cup of dashi.
I have been umami-bombing myself for like 15 years. It's not really all that mysterious and I think you're getting too in your head about it. I don't know how many people you're serving, but my initial thought would be to do 3-5 canapes each featuring a different classic umami-rich ingredient. Also have a shaker of MSG on the table and let guests taste it themselves by salting their finger or whatever. Parm, shiitake, miso, bonito flake, some crudo cured in kombu.... you know, whatever. But at that point you're basically just doing a mini-meal as hors d'oeuvres. That's how I'd want to do it, but I'm cheffy and pretentious. The easier route is just to start with dashi. Good dashi is simple and contains two classic sources of umami. It was by investigating the properties of dashi that umami was discovered in the first place. Start at the beginning. Free glutamic acid from kombu + isosinate from katsuobushi. Maybe serve two dashis, one classic and one with dried shiitake for an extra dose of guanylate. Or a whole dashi flight also featuring some crappy hon dashi or your own synthetic "dashi" constructed from MSG and magic powders.
I don't know, it's your dinner. Go nuts.