A One time burn is only achievable with a hard fork.
The HIVE DAO has 4450 HBD per day of funding and the STEEM DAO has 2808 SBD of funding, the difference is 1642. The idea has evolved to burn half of that, 821 per day as it is simple to send it to @null. No Hardfork required. If a future hard fork comes and the desire is to burn or if the needs of the DAO change, it can be easily adapted and accounted for. The number can be easy to increase or decrease, it's easy to turn off.