In response to your first question, about submission to boost:

The work you will have to do to get your library to the point of submission (assuming there's sufficient interest -- ask on the boost mailing list) includes:

• provide documentation in a manner consistent with boost's style and infrastructure
• ditto for tests
• ditto for build scripts
• modifying your code to use boost's portability macros/conventions
• getting it to build and perform well on platforms that you may have little or no experience with. I'm almost certain that you will be required to provide a version that builds cleanly on POSIX systems such as Linux and OS X.

Libraries undergoing submission are assigned a review manager. At some point, your library will be reviewed publicly on the boost mailing list. It can take quite a long time, depending on interest, to get your library through the review backlog.

Once the review starts, a lot of people will tear your code apart, often with little or no tact. So make sure you're comfortable with criticism. Revisions will probably have to be made to what you have now until people are satisfied. I haven't looked at your code, it's just what usually happens.

Assuming submission is successful, you will have to maintain your library for the foreseeable future. This will probably mean:

• familiarizing yourself with boost's release process
• responding to bug reports, even those where you don't have access to a similar system to that of the reporter. Of course, this is true wherever you 'publish' your code, but the exposure provided by boost will mean more bugs reported (even if they're invalid).
• there's talk on the mailing list of boost moving to a new version control/modularization/distribution system soon, too. So you will almost certainly have more maintenance work to do in future.
• More generally, you will have to stay up to date with boost's infrastructure changes, just to keep your library's head above water.

Exposure is nice of course, but submission to boost isn't a one-shot thing.

I haven't reviewed the code but am not seeing a reason why a hash table would need an interface any different than the standard interface of std::unordered_x regardless of its implementation, i.e. chaining vs. buckets. I would expect any library that is part of boost to use the std interface where one exists, if possible.

Dang. Impressive work. Building with Clang++ 4.2 I get the following warnings (with -Wall and -pedantic):

 
$ clang++ -Wall -pedantic -std=c++11 -stdlib=libc++ main.cpp
In file included from main.cpp:4:
./sentinel.hpp:107:77: warning: '&&' within '||' [-Wlogical-op-parentheses]
   bool operator() (const T& x, const T& y) const { return x == y || x != x && y != y; }
                                                                  ~~ ~~~~~~~^~~~~~~~~
./sentinel.hpp:107:77: note: place parentheses around the '&&' expression to silence this warning
   bool operator() (const T& x, const T& y) const { return x == y || x != x && y != y; }
                                                                            ^
                                                                     (               )

My argument would be that chained hash tables have the same algorithmic complexity but much better worst case performance. If you are worried about allocations you can plug in a pool allocator to the existing containers.

In my engine, my use cases for flat (and non-resizable) hash containers are mainly for:
* Ones that are "deserialized" from files, as I use in-place loading where the file is loaded into RAM and cast to a struct and is ready to use without parsing. My flat hash table uses only offsets (instead of pointers) and supports only POD types to allow for this.
* Similar to the above, if a hash table is required on a NUMA core, then it needs to be memcpy-able (and needs to keep working if memcpy'ed to a different address space), which is the same requirement as above.
* If a hash table requires simultaneous insertions from multiple cores, then a flat lock-free implementation is much simpler than one that uses traditional allocations.

^Given these quirky requirements, I don't think boost would care for my own implementation