But some lambics are sweetened with saccharine . . . maybe they’re not to your taste either, but there’s precedent.
With the supposed “brothiness” of it, perhaps MSG would work best in a rauchbier 
[quote]perhaps MSG would work best in a rauchbier
[/quote]
Or a dumpbier
Or a dumpbier
[/quote]
I’m still reserving judgment until I try it.
You could try mashing with dried porcini mushrooms. I always thought fresh guinness has a slight “soy sauce/umami” flavor, fwiw.
I’m pretty sure Randy Mosher has a recipe for a mushroom beer in Radical Brewing.
My first “experimental” beer is going to be a fish sauce and peanut stout, for the same reason as Guinness sometimes has that soy-y flavor.
I’ve used Randy’s concept several times to make a wee heavy that was “dry shroomed” with chanterelles. An absolutely fantastic beer. Randy calls his version “Nirvana”.
Sounds gross :) Let us know how you like it.