If I understand you correctly, you have four bridges in a loop, but you want to load balance. The problem is that STP will block one direction in order to prevent layer-2 looping.
An etherchannel must start on one device, and terminate on another device. You cannot channel from one device through two other devices to a fourth device.
You can channel from A to each of the two middle bridges, but it will be two separate links, one to each of the middle bridges. The same goes for B to the middle bridges.
LACP is a link protocol that cannot be sent through a bridge. It is only between the two devices on each end of the physical link. LACP frames are sent with the special
01:80:c2:00:00:02 multicast MAC address. The IEEE has reserved the
01:80:c2 OUI for link protocols, and frames with this special multicast OUI are not allowed to be forwarded from the interface on which they are received to another interface on 802.1D compliant bridges.