Ultimately, I think maybe the reason tf2maps.net frowns on non-completely-original maps is because, with this forum layout that you've got for posting maps, it's very easy to be overwhelmed. The site simply doesn't scale that well in terms of acting as a directory. So if you had too many 2fort_rain maps on here, you'd lose track. Having said that, I'm not really convinced that there _are_ a lot of modified Valve maps. On FPSBanana, mine is the only modified Lumberyard except for some kind of achievements map.
Forum layout has nothing to do with it. Infact it has recently been improved with an added map mode search ability and map thread prefix's; and before that, become its very own section modified specifically for map promotion. Rather than the "[WIP]" threads we had in the past. It's also not that people can't be arsed to search through 50 different 2fort_rain maps, it's that they can't be arsed to search through any 'converted' map at all. There's just no interest in it and each time one appears it offends anyone who released an original Beta for testing.
It all starts off as "a good idea", or "Wouldn't it be cool". When the fact is half these people have no idea what they are doing/talking about. They have no knowlegde of the game engine they are delving into and only imply the answer to the question asked by so many that do not occupy the custom content scene; as to whether this is accepted, normal practice, as "yes". When ettiquette surely tells you otherwise.
I've already given you the general consensus as to why most tf2maps.net members frown upon unoriginal material, and that is that it is A) offensive and B) unnecassery.
If you intended to learn something from it (and even if you didn't) there's still no reason to release it to the public.
Do some private testing, post up some questions and maybe some screenshots in the "mapping discussion" forum and put forward a thesis or synopsis of some kind and have a real discussion about it's validity.
I realise i'm cutting you off a little short here as you did include a question as to this very situation with your submission. But this is your answer. As Booj' said: You can submit your map to the database. But don't expect much of a response from the community beyond "make something original".
I'm not going to tell you your maps crap. Afterall, technically it's not your map. But please, for the love of god, make your own map based off of this inspirational idea.
As for the question as to how many re-hashed Valve maps there are in existance, the only answer is too many. Arena maps generally don't get re-hashed, infact most maps get made into arena. So you probably wouldn't find many custom versions of arena_lumberyard.