I would think this should be possible, but haven't tried it myself. I can clear up a few points, howerever:
Server 11 cannot host FileMaker 12 files.
Server advanced is not needed for custom web publishing Filemaker server can do this also and is much less expensive. Server Acvanced is needed for Instant web publishing and JDBC/ODBC.
FileMaker Pro Advanced (not server advanced) is a good thing to have for development, but offers no additional capabilities when it comes to hosting the file.
We just upgraded 1 license to desktop advanced and I may get it too for me as it would help with scriptmaster and script debugging (variable watch and also just commenting lines out). We already had server advanced for when we tried using filemaker as an odbc source so we just kept the same level in case we want to try again.
I know the 11 and 12 formats are different, but I would at least spin out a proof of concept. I may try just developing against the production server with a cloned file as I do not really need fmp server on my laptop. I already have mysql and sql 2005 on there and don't need to bog it down anymore.
Instant web publishing on standalone filemaker 12 desktop was up in no time, just can't get the file to be a source for custom web publishing (upload file to server and it works fine).