Don't wish to sound hard nosed, but just how far do you think it would be ok to change the rules???? FT rules are pretty clear. There may end up being some clarification etc at some point in terms of rules regarding this though following discussion.
Surely, if you are physically unable to shoot within the rules, then unfortunately you don't take part, simples.
It has been pointed out in relation to any number of other sports as well that this is the case. Make sure that the rules regarding this are explicitly clear for all and that they are enforced. If you are able to get round a course and sit, shoot in a sitting class. If you are unable to even do that, shoot a different discipline!
FT is to some extent a physical sport, there is no getting away from that, nor should we.