Hi transition - have I got this right? Are you planning on taking a group totalling around 19 people? If so that is WAY too many walkers for a wilderness area like Lake Rhona (or even Gordonvale). Safety and hygiene aside, think of the environmental impacts of a group that large simply through trampling boots.
There was a lot of angst on this forum about a large school group (no, a HUGE group) that went up the Arm River to Pelion Hut back in June. Check it out at
http://bushwalk.com/forum/viewtopic.php?f=5&t=4222&p=48010&hilit=Pelion+Hut+large+group#p48010 Even on a hardened track like the Overland, the group size limit is 13. So where does that leave groups like yours?
Firstly, good on you for seeking alternatives. What about you divide the group into 2 sub-groups - call them A and B. Then walk somewhere more hardened and only get together as a whole group at a hardened campsite. aljscott's idea of South Cape Rivulet has merit. You could camp all together at Cockle Creek for the first night (hardened, toilets etc). Then Group A heads down to South Cape Rivulet while Group B first does a day walk to Fishers Point and back. While Group A goes all the way to Sth Cape Rivulet to camp overnight, Group B follows later, only going as far as South Cape Bay (near Lion Rock). On the return trip they cross over, with Group A getting back and doing the Fishers Pt walk, and Group B going to South Cape Rivulet before returning to Cockle for a whole group session.
Mt Field might offer similar possibilities, and I'm sure others can think of suitable alternatives. But the key plea here is, don't take such a large group into a wilderness setting,
cheers
Peter