My main thoughts at the moment are that if someone states chicken, then vegan/vegetarians/pescatarians who want to enter the challenge should be allowed an equivalent substitution. So soya/TVP mince for chicken would not be allowed, but would be for a red meat switch. Tempeh would switch with anything, as would Seitan. But chicken flavoured quorn would only be allowed for a chicken or white meat substitution.
If Pumpkin is stated, then pumpkin should be the ingredient. If squash is stated, any hard skinned squash could be used, such as winter squash, pumpkin, butternut squash, but not marrow or courgette which although botanically fall under the same family are not really equivalent.
Dairy/Nut allergies are more interesting issues, as would be a gluten/lactose intolerance and we will just need to accept that milk is anything vaguely resembling a milk style drink. Same for yoghurt etc. Cheese, well there are substitutions but the flavours are not the same however, for vegans we adapt (as do those of us allergic to dairy).
But we can live, learn and modify and adapt as needed.
I'm beginning to think this is a minefield! We could debate endlessly about what substitutes should or shouldn't be allowed for fish, meat, chicken or game. Basic soya mince (not the one flavoured to taste meaty), could, for example be used in place of minced chicken or turkey. Quorn make products which are designed to replace minced meat (although obviously the chicken flavoured Quorn would only substitute for chicken). They also make 'sausages' which presumably could be used in a sausage casserole.
Rather than tying ourselves in knots over what should or shouldn't be 'allowed' as vegetarian subs, wouldn't it be easier to simple allow substitutes (for fish, fowl, or meat) at people's discretion. Its not as if we are overrun with recipes! And in the end, are we seriously going to 'disqualify' entries? Its up to the Judge to decide what wins, after all!