Hi Thomas, it could work. Moreover, the slave assignment should be deterministic, not random. In the documentation for the layer2+3 transmit hash policy:
https://www.kernel.org/doc/Documentation/networking/bonding.txt
the formula for the slave assignment is shown, so one could always be able to come up with specific IP addresses that would force the connections to use different links. Assuming (but this should be verified) that the slaves used by the switches to deliver traffic to the host are the same used by the host for outgoing traffic (this could be true only for 802.3ad), this should work both ways (in/out).
OR, one could make 2 active/backup bonds over 4 slaves and redirect connections across them using iptables NAT, and forget about 802.3ad completely 
Thanks, Giacomo
EDIT: some similar considerations about hashing algorithms: