Realistically speaking, I believe the most workable scenario would be to go for a Little Light sized piece (or maybe an "enhanced" LL) that is limited and numbered. I rather doubt that HL would be willing to assign a normal sequence number to a special piece. However, a specialty number would allow the piece you are looking for and still not mess with those who have a "complete" collection of all normal HL production.

HL may not be interested in handling individual distribution, but that can be worked around. We might also be expected to guarantee the sell out of a production run. (This is pretty much what I believe Lighthouse Depot has to do with their special pieces.)

A large LL sized piece would be affordable to produce, and the pricing could be kept reasonable. Personally, I would suggest that part of the income from a project of this type go to lighthouse preservation and a portion go to support the Forums.