Just throwing this out there, but I think the best contest to take a template from is the yearly AGA Aquascaping contest ( 2011 AGA Aquascaping Contest
) Not only do they allow tanks of any age/size, but they let the same tanks be submitted again if they have grown in better since the previous year. Obviously they have way more entrants and can make seperate size categories, but maybe down the road we can do the same. All they require is at least one main frontal picture, and some additional tank info.
The quality of their contests and submissions definitely make me want to start aquascaping, or at the very least make a paludarium to submit for next year. Whos to say a dendroboard contest wont increase our hobby's popularity by drawing in similar hobbyists?