I think you risk that your husband will refuse the next time you want him to do something that's important to you. So if you're willing to play that card on an event that is only 3 hours, then intervene with the MIL. Is she going to throw you under the bus with your husband? I'd personally bring ear plugs and suck it up. I'd probably go missing 15 minutes into the concert and come back for the last 30 with an excuse. Woops, I got turned around and couldn't find my way back
Or, you would not believe how long the lines are to use the women's bathroom!
Are they opening for someone or do they at least have a decent opener?