There are even earlier examples in English (pioneer and charioteer, for instance), but those didn't come from adding "-eer" to an activity; they were loanwords (http://en.wikipedia.org/wiki/Loanword) from other languages, like Old French, which used "-ier", "-eur" and "-aire" the way we use "-eer" today. (So charioteer, for instance, started out as charioteur.) Once the words were adopted by English, the spelling migrated to the form we know today.
Good points all. This is what I get for mouthing off from my phone instead of taking the time to think about what I'm saying. I will say though that given the continuing productivity of language, the fact that those earlier examples exist doesn't necessarily preclude my supposition that "X-eer" could see an uptick in neologistic usage - although again you're quite right that it wouldn't be "inchoate" in the sense of not being extant.
Mountaineer: first use 1600s (http://www.etymonline.com/index.php?term=mountaineer&allowed...)
Pamphleteer: first use 1640s (http://www.etymonline.com/index.php?term=pamphleteer&allowed...)
Auctioneer: first use 1708 (http://www.etymonline.com/index.php?term=auctioneer&allowed_...)
Profiteer: first use 1910-1915 (http://dictionary.reference.com/browse/profiteer)
There are even earlier examples in English (pioneer and charioteer, for instance), but those didn't come from adding "-eer" to an activity; they were loanwords (http://en.wikipedia.org/wiki/Loanword) from other languages, like Old French, which used "-ier", "-eur" and "-aire" the way we use "-eer" today. (So charioteer, for instance, started out as charioteur.) Once the words were adopted by English, the spelling migrated to the form we know today.