IM Jack Rudd wrote: So a Megafinal will, by construction, be a tournament such that a requirement for entry into it is eligibility to play for the relevant county.
I don't think Megafinals have ever been graded. It's only the last two stages of the UK Challenge that are. By the process of eliminating weak players, these usually contain players who already have grades.
The approach used by many local junior organisations is to run a series of rapidplays for local players. These would be structured as events for individuals and would presumably be classified by the ECF as requiring Silver membership. The original Farthing papers classified them in this way and was looking for an unrealistically high income from this subset.
I don't think they usually impose county only restrictions. Browsing a handful of players, you note that the most active ones play in events in several counties, for example all of Kent, Surrey and Sussex.
It's an interesting loophole that if a County Junior organiser restricts entry to the home county, then they can arbitrage the ECF's membership rules to possible financial advantage.
Juniors aren't members of chess clubs in the traditional sense of playing for that club in a local league. Rather they participate in activities run by the local organisations. These may mostly be open tournaments but also include the odd inter-county event including those nominally run by the ECF.