For gods sake, why not just release a set amount of beta keys in rounds to anyone that has applied to the "beta sign up", eg:
1000 keys in Feb
1000 keys in March
1000 keys in April
ect
There's no need to exclusively give keys to "modders" and debate what defines a "modder".
No one is selected with a bias, its all random.
Who cares if "modders" don't get the first access, so long as some get in by chance or by quota during the beta, all is well.
The lucky modders get to try out the mod kit, API changes ect, give feedback and perhaps start building their mod how ever small or large it may be.
Also, why should modders exclusively gain access first? Even if they do and they try to start working on their project, there's a high chance that updates to the game will be frequent and some of those changes in the game (back end) may be significant enough to set the modders back even further #pain in the ass.
All that matters during the beta phase is feedback from the community in general (casual, hardcore,modders,ect). That's where my first suggestion comes in (x amount of keys released in rounds to people that have signed up to the beta).
Edit: who knows, perhaps by this random selection process new modders may arise who are quite proficient at what they do and may not only produce new content, but that of which is better than what we have already seen.