should be possible with steem-python:
(untested code, just by heart!!!)

#!/usr/bin/python
from steembase import operations
from steem.transactionbuilder import TransactionBuilder

sender = "pawsdog"
receivers = ['rec1', 'rec2', 'rec3'] # fill your list here
ops = []
for rec in receivers:
    op = operations.Transfer(
    **{"from": sender,
          "to": rec,
          "amount": "0.001 SBD",
          "memo": "this is for you, %s" % (rec)
        }
    )
    ops.extend(op)

tx = TransactionBuilder()
tx.appendOps(ops)
tx.appendWif("put your active key here")
tx.sign()
tx.broadcast()

This groups transfers into the same block. I'm not sure how many receivers you can add there without exceeding block/transaction size limits but breaking this down into multiples of like 50 or 100 transfers at once should not be a big issue.