Thats not having the radiator in the chain but letting water find its own level through the heatercore. Like this.
http://lardarse78.tripod.com/look.htm
Can even chain more than 1 radiator thru 2 reservoirs in addition to the present radiator you are using. Maybe even saw off two end tanks and gooping the two ends to the 2 res.
So you can use more than 1 rad without extra pump and not affecting flow rate.
Do you think it'll work?