We wish to peer with some members of an IXP that all support IPv6 peering. We do not wish to publicly peer on the IXP. Is it possible to save ourselves the trouble of getting a /24 block of v4's and skip straight to a block of v6s to peer with these members?
You should ask the members if they want to peer with you in this fashion. There are no rules agains peering IPv6-only, it all depends on the peering policy your partner has.