This is possible by using the latest RC and protocol-mode=none.
Are you sure you want to use LACP on links that are bridged? There can be some throughput issues since LACP requires both links to be with the same speed.
I would guess that you are trying to use Wireless links, which can change the Wireless rate at any time, you should consider using other bonding modes.
You can read about an issue that you are going to have when using Wireless links in a bond here:
https://wiki.mikrotik.com/wiki/Manual:L ... less_links
If you are using CRS317, which is capable of offloading to the switch chip LACP interfaces, then you will have to give up hardware support for the bonding interface because of the protocol limitations described in the link.
LACPDUs are not meant to be forwarded for a reason, take precaution when forwarding them.