Yes. Robert Cartaino confirmed* that it is possible for Area 51 to contact those who supported the other proposals, but that apparently won't happen right now. We are, it seems, "doing really well."
In light of Robert's information, perhaps this question should be updated. It might be rephrased, "What are the requirements this proposal needs to meet in order for Area 51 to send an e-mail to the former initiative(s) supporters?"
The two now closed previous efforts are listed below:
(1) The fairly well known "Genealogy" proposal
(2) The lesser known "Geneology" proposal
A few items follow that put the rationale for the revised question in context.
(a) The "Geneology" proposal was initiated in June of this year. The proposal page (link above) reports it was closed as a duplicate because it "would drain audience from [the "genealogy"] site."
(b) Give or take a month later, the "Genealogy" proposal was closed because, it was "not a real proposal"; it would "not result in a useful productive or viable Q&A site." At the time of closure, it had had 44 committers with 200+ rep on other sites.
(c) Give or take a month after "Genealogy" was closed, Justin restarted this proposal, now "Genealogy & Family History." When compared to the most recently closed initiative we have only have only two more committers with 200+ other rep (as of this writing).
I hope the folks at Area 51 will reconsider and support the group by sending the e-mail to those who supported the other proposals.
Please give us a chance to work together to carry this proposal closer to the goal line. We need to work together to develop strong support from those with experience on SE.
*Comment under the first answer to "Is a notification sent out to tell followers ..."
Revised: Sept 14, 2012
Update: Sept 19, 2012--We are now at 56% commitment. @RobertCartaino, perhaps now StackExchange will notice those who participated in the prior initiatives? If not then, as above, what are the requirements for those notices to be sent?