As far as I remember it is supported with algorithm "custom", even non-terminated MPLS packets are hashing with the following rules:
For L3 payload (i.e. not VPLS or VPWS), the IP source and dest addresses and the labels are used for hashing.
If the payload is VPLS/VPWS, only the labels are used for hashing. Since these have an inner L2 frame, the hardware can’t look that deep inside to see the L3 fields.