If I hold a US stock in a custodian brokerage like FSMOne, will I get invited to the US company's annual general meeting?

You do not own the stocks, all invitation to be sent to the custodian instead. You can express your interest if you are interested to attend and they will send a proxy form to you.

You definitely should contact FSMOne to register you for the AGM as a proxy especially if you hold Berkshire Hathaway stocks, I would feel that their AGM is worth attending! Not everyday you get to hear the oracle of omaha sharing his experiences and outlook for the coming year