As you are not a registered owner of the aircraft you have no legal right to claim part ownership. ANO Art 6. This will have no effect on the insurance if you are nominated as a pilot of the aircraft however; there may be an issue if the aircraft is maintained to private standards and as you are not legally a joint owner, then any payment for the use or hire of the aircraft would be illegal. If the aircraft is maintained to commercial transport standards then there is no problem with paying to use the aircraft. Issues related to Flight Training are covered in
AIC 1/2011