if you can easily switch back and forth between the saddle and bench seat, that is different, and you can likely hang the seat wherever you want to, assuming you don't loose too much stability.  Keep in mind that if you will be kneeling from that seat occasionally, then it needs to be high enough to avoid foot entrapment under it.    They make ankle supports for ww boats - kind of a half cylinder with the flat down and crosswise to support the ankle - you might look arouns for something like that to help support the ankles when kneeling.  Mohawk sells some "ankle blocks" that are wedge shaped so you can tilt your feet at the ankles - that is another option ( see mohawk canoes.com / outfitting store/  ankle blocks ) - the mini cell blocks are easy to install, just takes contact cement you can buy anywhere